info: Report actual checked-out HEAD revision

When `repo info` runs, the reported "Current revision" is resolved using
the manifest's target branch tracking ref (e.g. refs/remotes/goog/main).

Introduce Project.GetHeadRevisionId(), which gets the checked-out HEAD
commit in the worktree, and use it in `repo info` with a fallback to the
old behavior if the project is not checked out.

Bug: 526685287
Change-Id: I72280ce27daa210cada27d722a94e365644f06e0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/599481
Reviewed-by: Brian Gan <brgan@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>

subcmds/info.py

@@ -196,7 +196,8 @@ class Info(PagedCommand):
         data = {
             "name": project.name,
             "mount_path": project.worktree,
-            "current_revision": project.GetRevisionId(),
+            "current_revision": project.GetHeadRevisionId()
+            or project.GetRevisionId(),
             "manifest_revision": project.revisionExpr,
             "local_branches": list(project.GetBranches()),
         }
@@ -273,7 +274,7 @@ class Info(PagedCommand):
         out.nl()
 
         heading("Current revision: ")
-        headtext(project.GetRevisionId())
+        headtext(project.GetHeadRevisionId() or project.GetRevisionId())
         out.nl()
 
         currentBranch = project.CurrentBranch