Handle grep output

- Handle standard filepath:code and filepath:line_number:code output
  as produced by `git grep`, `rg -H`, `grep -H`, etc (with -n for line
  numbers).

- Retain the match highlighting as produced by the grep tool, and
  expose it in delta's color output styled with grep-match-style.
  (Note that --color=always is needed to retain the color if piping
  into delta, but not for `git grep` when delta is configured as git's
  pager)

- Special handling of -p, and -W options of `git grep`: these display
  the function context in which the matches occur.

- `navigate` keybindings jump between match function contexts under
  `git grep -p` and between matching lines under `git grep -W`.

Thanks @zachriggle for the proposal.
Fixes #769

README.md

@@ -23,6 +23,7 @@
     show = delta
     log = delta
     blame = delta
+    grep = delta
     reflog = delta
 
 [interactive]
@@ -64,6 +65,7 @@ Code evolves, and we all spend time studying diffs. Delta aims to make this both
   - [Choosing colors (styles)](#choosing-colors-styles)
   - [Line numbers](#line-numbers)
   - [Side-by-side view](#side-by-side-view)
+  - [grep](#grep)
   - [git blame](#git-blame)
   - ["Features": named groups of settings](#features-named-groups-of-settings)
   - [Custom themes](#custom-themes)
@@ -152,6 +154,7 @@ Here's what `git show` can look like with git configured to use delta:
 - Git style strings (foreground color, background color, font attributes) are supported for >20 stylable elements
 - Side-by-side view with line-wrapping
 - Line numbering
+- Handles grep output with file paths from `rg`, `git grep`, `grep`, etc
 - `diff-highlight` and `diff-so-fancy` emulation modes
 - Stylable box/line decorations to draw attention to commit, file and hunk header sections.
 - Support for Git's `--color-moved` feature.
@@ -404,6 +407,13 @@ In contrast, the long replacement line in the right panel overflows by almost an
 For control over the details of line wrapping, see `--wrap-max-lines`, `--wrap-left-symbol`, `--wrap-right-symbol`, `--wrap-right-percent`, `--wrap-right-prefix-symbol`, `--inline-hint-style`.
 Line wrapping was implemented by @th1000s.
 
+### grep
+
+To use with `git grep`, set delta as the pager for `grep` in the `[pager]` section of your gitconfig. See the example at the [top of the page](#get-started).
+Output from other grep tools can be piped to delta: use `rg -Hn --color=always`, `grep -Hn --color=always`, etc.
+Grep matches are styled with `grep-match-style`, `grep-match-style`, and `grep-match-line-number-style`.
+Note that `git grep` can display the "function context" for matches and that delta handles this output specially: see the `-p` and `-W` options of `git grep`.
+
 ### git blame
 
 Set delta as the pager for `blame` in the `[pager]` section of your gitconfig. See the example at the [top of the page](#get-started).

src/cli.rs

@@ -441,6 +441,21 @@ pub struct Opt {
     )]
     pub blame_timestamp_format: String,
 
+    #[structopt(long = "grep-match-style")]
+    /// Style (foreground, background, attributes) for grep matches. See STYLES
+    /// section. Defaults to plus-style.
+    pub grep_match_style: Option<String>,
+
+    #[structopt(long = "grep-match-file-style")]
+    /// Style (foreground, background, attributes) for grep match file paths. See STYLES
+    /// section. Defaults to hunk-header-file-path-style.
+    pub grep_match_file_style: Option<String>,
+
+    #[structopt(long = "grep-match-line-number-style")]
+    /// Style (foreground, background, attributes) for grep match line numbers. See STYLES
+    /// section. Defaults to hunk-header-line-number-style.
+    pub grep_match_line_number_style: Option<String>,
+
     /// Default language used for syntax highlighting when this cannot be
     /// inferred from a filename. It will typically make sense to set this in
     /// per-repository git config (.git/config)

src/config.rs

@@ -78,6 +78,9 @@ pub struct Config {
     pub git_config: Option<GitConfig>,
     pub git_minus_style: Style,
     pub git_plus_style: Style,
+    pub grep_match_file_style: Style,
+    pub grep_match_line_number_style: Style,
+    pub grep_match_style: Style,
     pub hunk_header_file_style: Style,
     pub hunk_header_line_number_style: Style,
     pub hunk_header_style_include_file_path: bool,
@@ -216,6 +219,22 @@ impl From<cli::Opt> for Config {
             _ => *style::GIT_DEFAULT_PLUS_STYLE,
         };
 
+        let grep_match_style = if let Some(s) = opt.grep_match_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            plus_emph_style
+        };
+        let grep_match_file_style = if let Some(s) = opt.grep_match_file_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            hunk_header_file_style
+        };
+        let grep_match_line_number_style = if let Some(s) = opt.grep_match_line_number_style {
+            Style::from_str(&s, None, None, opt.computed.true_color, false)
+        } else {
+            hunk_header_line_number_style
+        };
+
         let blame_palette = make_blame_palette(opt.blame_palette, opt.computed.is_light_mode);
 
         let file_added_label = opt.file_added_label;
@@ -282,6 +301,9 @@ impl From<cli::Opt> for Config {
             file_style,
             git_config: opt.git_config,
             git_config_entries: opt.git_config_entries,
+            grep_match_file_style,
+            grep_match_line_number_style,
+            grep_match_style,
             hunk_header_file_style,
             hunk_header_line_number_style,
             hunk_header_style,

src/delta.rs

@@ -23,6 +23,7 @@ pub enum State {
     SubmoduleLog, // In a submodule section, with gitconfig diff.submodule = log
     SubmoduleShort(String), // In a submodule section, with gitconfig diff.submodule = short
     Blame(String, Option<String>), // In a line of `git blame` output (commit, repeat_blame_line).
+    Grep(String, Option<String>), // In a line of `git grep` output (file, repeat_grep_line).
     Unknown,
     // The following elements are created when a line is wrapped to display it:
     HunkZeroWrapped,  // Wrapped unchanged line
@@ -121,6 +122,7 @@ impl<'a> StateMachine<'a> {
                 || self.handle_submodule_short_line()?
                 || self.handle_hunk_line()?
                 || self.handle_blame_line()?
+                || self.handle_grep_line()?
                 || self.should_skip_line()
                 || self.emit_line_unchanged()?;
         }

src/handlers/file_meta.rs

@@ -210,7 +210,7 @@ fn get_file_extension_from_file_meta_line_file_path(path: &str) -> Option<&str>
 }
 
 /// Attempt to parse input as a file path and return extension as a &str.
-fn get_extension(s: &str) -> Option<&str> {
+pub fn get_extension(s: &str) -> Option<&str> {
     let path = Path::new(s);
     path.extension()
         .and_then(|e| e.to_str())