project: allow CurrentBranch to return None on errors

If the repo client checkout is in an incomplete sync state, the work
git repo might be in a bad way.  Turn errors parsing HEAD into None
since callers of CurrentBranch already need to account for it.

Change-Id: Ia7682e29ef4182006b1fb5f5e57800f8ab67a9f4
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239239
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

project.py

@@ -866,10 +866,17 @@ class Project(object):
   @property
   def CurrentBranch(self):
     """Obtain the name of the currently checked out branch.
-       The branch name omits the 'refs/heads/' prefix.
-       None is returned if the project is on a detached HEAD.
+
+    The branch name omits the 'refs/heads/' prefix.
+    None is returned if the project is on a detached HEAD, or if the work_git is
+    otheriwse inaccessible (e.g. an incomplete sync).
     """
-    b = self.work_git.GetHead()
+    try:
+      b = self.work_git.GetHead()
+    except NoManifestException:
+      # If the local checkout is in a bad state, don't barf.  Let the callers
+      # process this like the head is unreadable.
+      return None
     if b.startswith(R_HEADS):
       return b[len(R_HEADS):]
     return None