status: Show ahead/behind info for local branches

When viewing `repo status`, it is difficult to distinguish between branches that have active unpushed changes and stale branches that are fully synced. Previously, developers had to run commands like `repo forall -c "git status"` to see their ahead/behind counts.

This change updates Project.PrintWorkTreeStatus to automatically calculate and display the number of commits a branch is ahead and/or behind its upstream tracking branch. We use `git rev-list --left-right --count` to fetch this information natively and efficiently.

If the branch is completely synced with upstream, no extra text is shown.

Added tests for ahead-only, behind-only, diverged, no-tracking, and fully-synced branch states.

Bug: 319412954
Change-Id: I23879b2d472c7a7e11d01b565428a84b1b4f09c1
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/602423
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Brian Gan <brgan@google.com>
Commit-Queue: Brian Gan <brgan@google.com>

project.py

@@ -996,11 +996,32 @@ class Project:
             out.nl()
             return "DIRTY"
 
-        branch = self.CurrentBranch
-        if branch is None:
+        branch_name = self.CurrentBranch
+        if branch_name is None:
             out.nobranch("(*** NO BRANCH ***)")
         else:
-            out.branch("branch %s", branch)
+            branch_obj = self.GetBranch(branch_name)
+            ahead_behind = ""
+            try:
+                local_merge = branch_obj.LocalMerge
+                if local_merge:
+                    left_right = self.work_git.rev_list(
+                        "--left-right",
+                        "--count",
+                        f"{local_merge}...{R_HEADS}{branch_name}",
+                    )
+                    left, right = left_right[0].split()
+                    behind = int(left)
+                    ahead = int(right)
+                    if ahead and behind:
+                        ahead_behind = f" [ahead {ahead}, behind {behind}]"
+                    elif ahead:
+                        ahead_behind = f" [ahead {ahead}]"
+                    elif behind:
+                        ahead_behind = f" [behind {behind}]"
+            except GitError:
+                pass
+            out.branch("branch %s%s", branch_name, ahead_behind)
         out.nl()
 
         if rb: