Differentiate between Executable and Program

It's somewhat confusing, but it seems sometimes `git` uses a shell, but other
times it doesn't. And that is differentiate by the type system.

It would still have to be (re)validated at some point if this is actually the case,
but I guess there was a reason for this in the first place.

gix/src/config/tree/sections/core.rs

@@ -23,7 +23,7 @@ impl Core {
     pub const DISAMBIGUATE: Disambiguate =
         Disambiguate::new_with_validate("disambiguate", &config::Tree::CORE, validate::Disambiguate);
     /// The `core.editor` key.
-    pub const EDITOR: keys::Executable = keys::Executable::new_executable("editor", &config::Tree::CORE);
+    pub const EDITOR: keys::Program = keys::Program::new_program("editor", &config::Tree::CORE);
     /// The `core.fileMode` key.
     pub const FILE_MODE: keys::Boolean = keys::Boolean::new_boolean("fileMode", &config::Tree::CORE);
     /// The `core.ignoreCase` key.
@@ -60,10 +60,10 @@ impl Core {
         .with_environment_override("GIT_ASKPASS")
         .with_note("fallback is 'SSH_ASKPASS'");
     /// The `core.excludesFile` key.
-    pub const EXCLUDES_FILE: keys::Executable = keys::Executable::new_executable("excludesFile", &config::Tree::CORE);
+    pub const EXCLUDES_FILE: keys::Path = keys::Path::new_path("excludesFile", &config::Tree::CORE);
     /// The `core.attributesFile` key.
-    pub const ATTRIBUTES_FILE: keys::Executable =
-        keys::Executable::new_executable("attributesFile", &config::Tree::CORE)
+    pub const ATTRIBUTES_FILE: keys::Path =
+        keys::Path::new_path("attributesFile", &config::Tree::CORE)
             .with_deviation("for checkout - it's already queried but needs building of attributes group, and of course support during checkout");
     /// The `core.sshCommand` key.
     pub const SSH_COMMAND: keys::Executable = keys::Executable::new_executable("sshCommand", &config::Tree::CORE)

gix/src/config/tree/sections/diff.rs

@@ -19,10 +19,10 @@ impl Diff {
     pub const RENAMES: Renames = Renames::new_renames("renames", &config::Tree::DIFF);
 
     /// The `diff.<driver>.command` key.
-    pub const DRIVER_COMMAND: keys::String = keys::String::new_string("command", &config::Tree::DIFF)
+    pub const DRIVER_COMMAND: keys::Program = keys::Program::new_program("command", &config::Tree::DIFF)
         .with_subsection_requirement(Some(SubSectionRequirement::Parameter("driver")));
     /// The `diff.<driver>.textconv` key.
-    pub const DRIVER_TEXTCONV: keys::String = keys::String::new_string("textconv", &config::Tree::DIFF)
+    pub const DRIVER_TEXTCONV: keys::Program = keys::Program::new_program("textconv", &config::Tree::DIFF)
         .with_subsection_requirement(Some(SubSectionRequirement::Parameter("driver")));
     /// The `diff.<driver>.algorithm` key.
     pub const DRIVER_ALGORITHM: Algorithm =