git: add rust opt-in

With v2.52.0, git now includes the option to use a rust implementation
of some core logic [1]. Using rust/cargo as part of the build process is
currently still optional, though expected to become mandatory in Git
v3.0.

In preparation, introduce cargo/rustc into the Git package build. This
is disabled by default and enabled via a new `rustSupport` argument to
mirror the fact that this is opt-in in Git itself.

[1]: lore.kernel.org/git/20251002-b4-pks-rust-breaking-change-v8-0-3a89fd5b1ce7@pks.im/

pkgs/by-name/gi/git/package.nix

@@ -51,6 +51,9 @@
   deterministic-host-uname, # trick Makefile into targeting the host platform when cross-compiling
   doInstallCheck ? !stdenv.hostPlatform.isDarwin, # extremely slow on darwin
   tests,
+  rustSupport ? false,
+  cargo,
+  rustc,
 }:
 
 assert osxkeychainSupport -> stdenv.hostPlatform.isDarwin;
@@ -186,6 +189,10 @@ stdenv.mkDerivation (finalAttrs: {
   ++ lib.optionals withLibsecret [
     glib
     libsecret
+  ]
+  ++ lib.optionals rustSupport [
+    cargo
+    rustc
   ];
 
   # required to support pthread_cancel()
@@ -234,7 +241,9 @@ stdenv.mkDerivation (finalAttrs: {
   # acceptable version.
   #
   # See https://github.com/Homebrew/homebrew-core/commit/dfa3ccf1e7d3901e371b5140b935839ba9d8b706
-  ++ lib.optional stdenv.hostPlatform.isDarwin "TKFRAMEWORK=/nonexistent";
+  ++ lib.optional stdenv.hostPlatform.isDarwin "TKFRAMEWORK=/nonexistent"
+  # Starting with future Git version 3.0.0, rust will be mandatory. For now, it's optional.
+  ++ lib.optional rustSupport "WITH_RUST=YesPlease";
 
   disallowedReferences = lib.optionals (stdenv.buildPlatform != stdenv.hostPlatform) [
     stdenv.shellPackage