project: Refactor GetHead to use symbolic-ref first

Simplify branch resolution and optimize unborn branch detection by
prioritizing symbolic-ref over rev-parse.

Change-Id: Ic62dcb87cd051dafb00d520b1157be2e32abc2ab
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/563222
Reviewed-by: Nasser Grainawi <nasser.grainawi@oss.qualcomm.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>

project.py

@@ -3964,30 +3964,14 @@ class Project:
         def GetHead(self):
             """Return the ref that HEAD points to."""
             try:
-                symbolic_head = self.rev_parse("--symbolic-full-name", HEAD)
-                if symbolic_head == HEAD:
-                    # Detached HEAD. Return the commit SHA instead.
-                    return self.rev_parse(HEAD)
-                return symbolic_head
-            except GitError as e:
-                # `git rev-parse --symbolic-full-name HEAD` will fail for unborn
-                # branches, so try symbolic-ref before falling back to raw file
-                # parsing.
-                try:
-                    p = GitCommand(
-                        self._project,
-                        ["symbolic-ref", "-q", HEAD],
-                        bare=True,
-                        gitdir=self._gitdir,
-                        capture_stdout=True,
-                        capture_stderr=True,
-                        log_as_error=False,
-                    )
-                    if p.Wait() == 0:
-                        return p.stdout.rstrip("\n")
-                except GitError:
-                    pass
+                return self.symbolic_ref("-q", HEAD, log_as_error=False)
+            except GitError:
+                pass
 
+            try:
+                # If symbolic-ref fails, try to treat as detached HEAD.
+                return self.rev_parse(HEAD)
+            except GitError as e:
                 logger.warning(
                     "project %s: unparseable HEAD; trying to recover.\n"
                     "Check that HEAD ref in .git/HEAD is valid. The error "