feat: add key for diff.external.

That way it's conceivable that applications correctly run either
a configured external diff tool, or one that is configured on a
per diff-driver basis, while being allowed to fall back to
a built-in implementation as needed.

Author: Byron

gix/src/config/cache/init.rs

@@ -530,6 +530,16 @@ fn apply_environment_overrides(
                 (env(key), key.name)
             }],
         ),
+        #[cfg(feature = "blob-diff")]
+        (
+            "diff",
+            None,
+            git_prefix,
+            &[{
+                let key = &config::tree::Diff::EXTERNAL;
+                (env(key), key.name)
+            }],
+        ),
     ] {
         let mut section = env_override
             .new_section(section_name, subsection_name)

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

@@ -32,6 +32,10 @@ impl Diff {
     /// The `diff.<driver>.binary` key.
     pub const DRIVER_BINARY: Binary = Binary::new_with_validate("binary", &config::Tree::DIFF, validate::Binary)
         .with_subsection_requirement(Some(SubSectionRequirement::Parameter("driver")));
+
+    /// The `diff.external` key.
+    pub const EXTERNAL: keys::Program =
+        keys::Program::new_program("external", &config::Tree::DIFF).with_environment_override("GIT_EXTERNAL_DIFF");
 }
 
 impl Section for Diff {
@@ -48,6 +52,7 @@ impl Section for Diff {
             &Self::DRIVER_TEXTCONV,
             &Self::DRIVER_ALGORITHM,
             &Self::DRIVER_BINARY,
+            &Self::EXTERNAL,
         ]
     }
 }

gix/tests/gix-init.rs

@@ -50,7 +50,8 @@ mod with_overrides {
             .set("GIT_ICASE_PATHSPECS", "pathspecs-icase")
             .set("GIT_TERMINAL_PROMPT", "42")
             .set("GIT_SHALLOW_FILE", "shallow-file-env")
-            .set("GIT_NAMESPACE", "namespace-env");
+            .set("GIT_NAMESPACE", "namespace-env")
+            .set("GIT_EXTERNAL_DIFF", "external-diff-env");
         let mut opts = gix::open::Options::isolated()
             .cli_overrides([
                 "http.userAgent=agent-from-cli",
@@ -234,6 +235,8 @@ mod with_overrides {
             ("gitoxide.http.verbose", "true"),
             ("gitoxide.allow.protocolFromUser", "file-allowed"),
             ("core.useReplaceRefs", "no-replace"),
+            #[cfg(feature = "blob-diff")]
+            ("diff.external", "external-diff-env"),
             ("gitoxide.objects.replaceRefBase", "refs/replace-mine"),
             ("gitoxide.committer.nameFallback", "committer name"),
             ("gitoxide.committer.emailFallback", "committer email"),