feat(git): support shallow fetch for Git CLI backend (#16156)

### What does this PR try to resolve?

Fixes #14956

### How to test and review this PR?

This is done with some test generalization, so that we can test more
backend interop in the future when libgit2 support `file://` protocol
shallow clones.

In the future we should add container tests for at least gitoxide and
libgit2 backend to ensure they really work with https protocol

Author: weihanglo

src/cargo/sources/git/utils.rs

@@ -1030,8 +1030,9 @@ pub fn fetch(
         }
     }
 
+    debug!("doing a fetch for {remote_url}");
     let result = if let Some(true) = gctx.net_config()?.git_fetch_with_cli {
-        fetch_with_cli(repo, remote_url, &refspecs, tags, gctx)
+        fetch_with_cli(repo, remote_url, &refspecs, tags, shallow, gctx)
     } else if gctx.cli_unstable().gitoxide.map_or(false, |git| git.fetch) {
         fetch_with_gitoxide(repo, remote_url, refspecs, tags, shallow, gctx)
     } else {
@@ -1075,15 +1076,22 @@ fn fetch_with_cli(
     url: &str,
     refspecs: &[String],
     tags: bool,
+    shallow: gix::remote::fetch::Shallow,
     gctx: &GlobalContext,
 ) -> CargoResult<()> {
+    debug!(target: "git-fetch", backend = "git-cli");
+
     let mut cmd = ProcessBuilder::new("git");
     cmd.arg("fetch");
     if tags {
         cmd.arg("--tags");
     } else {
         cmd.arg("--no-tags");
     }
+    if let gix::remote::fetch::Shallow::DepthAtRemote(depth) = shallow {
+        let depth = 0i32.saturating_add_unsigned(depth.get());
+        cmd.arg(format!("--depth={depth}"));
+    }
     match gctx.shell().verbosity() {
         Verbosity::Normal => {}
         Verbosity::Verbose => {
@@ -1126,6 +1134,8 @@ fn fetch_with_gitoxide(
     shallow: gix::remote::fetch::Shallow,
     gctx: &GlobalContext,
 ) -> CargoResult<()> {
+    debug!(target: "git-fetch", backend = "gitoxide");
+
     let git2_repo = repo;
     let config_overrides = cargo_config_to_gitoxide_overrides(gctx)?;
     let repo_reinitialized = AtomicBool::default();
@@ -1234,7 +1244,8 @@ fn fetch_with_libgit2(
     shallow: gix::remote::fetch::Shallow,
     gctx: &GlobalContext,
 ) -> CargoResult<()> {
-    debug!("doing a fetch for {remote_url}");
+    debug!(target: "git-fetch", backend = "libgit2");
+
     let git_config = git2::Config::open_default()?;
     with_fetch_options(&git_config, remote_url, gctx, &mut |mut opts| {
         if tags {