Improve support for submodule diffs

[phil-blain]

First, great project!!

I noticed that the support for diffs including submodule changes could be improved. Some examples :

1. Let's say I have a project with a submodule ("brooklyn"), and I modify a file in the submodule :

   $ git status
   On branch add-sub
   Changes not staged for commit:
     (use "git add <file>..." to update what will be committed)
     (use "git checkout -- <file>..." to discard changes in working directory)
     (commit or discard the untracked or modified content in submodules)
   
       modified:   brooklyn (modified content)
   
   no changes added to commit (use "git add" and/or "git commit -a")

   the output of git diff, using diff-so-fancy is

   $ git diff
   ────────────────────────────────────────────────────────────────────────────────────
   modified: brooklyn
   ────────────────────────────────────────────────────────────────────────────────────
   @ brooklyn:1 @
   Subproject commit 30f3d2754a98670d57d142193ba51af3038555af
   Subproject commit 30f3d2754a98670d57d142193ba51af3038555af-dirty
   

   that's good, but it could be useful to have the header be

   modified: brooklyn (modified content)
   

   to reflect the output of git status. SImilarly, if new commits were added to the submodule, the header could indicate it : modified: brooklyn (new commits)

2. I usually set diff.submodule log. With this setting, the brooklyn submodule looses its header with (or without) diff-so-fancy :

   $ git diff --submodule=log
   Submodule brooklyn contains modified content
   $ git -P diff --submodule=log
   Submodule brooklyn contains modified content
   

3. Same thing with git diff --submodule=diff, the submodule itself doesn't have a header but its modified files do have one :

   Submodule brooklyn contains modified content
   ────────────────────────────────────────────────────────────────────────────────────
   modified: brooklyn/README.md
   ────────────────────────────────────────────────────────────────────────────────────
   @ brooklyn/README.md:93 @ Useful topics include:
   # diff here
   

[scottchiefbaker]

I've never used sub-modules. If you can submit a test case that adequately recreates this issue in a reliable and repeatable manner I can investigate this new feature..

[abrioy]

I'll try to provide a test case for you using both values of diff.submodule.

I'm using a demo repo from github where the css and js folder are submodules.

git clone --recurse-submodules https://github.com/githubtraining/example-dependency
cd example-dependency

cd js
# Making some changes in a submodule
echo -e "\n\n Committed test diff in README" >> README.md
git commit -am "Update README"
echo -e "\n\n Dirty test diff in README" >> README.md

cd ..
# This diff shows the committed changes to the submodule
git config diff.submodule log
git diff 

# This one shows the full diff
git config diff.submodule diff
git diff

Attached are the patch files for both examples :

example1.patch.txt
example2.patch.txt

Thank you for this tool, A header for the submodules would be a great addition.

[phil-blain]

@abrioy I've been trying https://github.com/dandavison/delta and it does add a submodule header.

[scottchiefbaker]

I will investigate this now that I have some other stuff off my plate.