git-cache: contains_revision returns False if sentient file exists

If git_cache.populate() gets interrupted midway, it can possibly leave
a git mirror in a state where only some commits are available. Cloning
such a repo results in an empty repository and causes subsequent git
checkout to fail.

If the sentient file exists, it indicates that the bootstrapping
process was interrupted. This CL updates git_cache.contains_revision()
such that it returns False if the sentient file exists. Then,
the caller can call mirror.populate() to re-populate the repo/rev.

Bug: 41488725
Change-Id: I24fd24e300f5a9f9349589496d6b5f3dacf71fd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6491733
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

git_cache.py

@@ -317,6 +317,14 @@ class Mirror(object):
         if not self.exists():
             return False
 
+        # If the cache population is in progress, this will raise LockError().
+        with lockfile.lock(self.mirror_path, timeout=0):
+            # If the sentinent file exists at this point, it indicates that
+            # the bootstrapping process was interrupted, leaving the cache
+            # entries in a bad state.
+            if os.path.isfile(self._init_sentient_file):
+                return False
+
         if sys.platform.startswith('win'):
             # Windows .bat scripts use ^ as escape sequence, which means we have
             # to escape it with itself for every .bat invocation.