find-releases: handle untagged merges

At the moment, this doesn't work too well for something that was just merged to
45.

[(b912ace...)]scottmg@scottmg:/work/cr/src$ git find-releases 55b4c95889d6a72e52bc72702580a62f04f35777
commit 55b4c95889d6a72e52bc72702580a62f04f35777 was:
  initially in 47.0.2501.0
  merged to undefined (as de54b58d404a0cdf01639ae7260ef10bbb843ed9)
  merged to 46.0.2490.19 (as b4ba1071baaabc92cb5a2ca89e238c478eca0075)

("undefined" being the problem). This is because:

[(b912ace...)]scottmg@scottmg:/work/cr/src$ git name-rev --tags de54b58d404a0cdf01639ae7260ef10bbb843ed9
de54b58d404a0cdf01639ae7260ef10bbb843ed9 undefined

But,

[(b912ace...)]scottmg@scottmg:/work/cr/src$ cat .git/refs/remotes/branch-heads/2454
de54b58d404a0cdf01639ae7260ef10bbb843ed9

So, if name-rev --tags says it's not yet tagged, rather than just printing
'undefined', at least say which branch it was merged to. As in:

[(b912ace...)]scottmg@scottmg:/work/cr/src$ git find-releases 55b4c95889d6a72e52bc72702580a62f04f35777
commit 55b4c95889d6a72e52bc72702580a62f04f35777 was:
  initially in 47.0.2501.0
  merged to branch-heads/2454 [untagged] (as de54b58d404a0cdf01639ae7260ef10bbb843ed9)
  merged to 46.0.2490.19 (as b4ba1071baaabc92cb5a2ca89e238c478eca0075)


R=dpranke@chromium.org,primiano@chromium.org

Review URL: https://codereview.chromium.org/1325233005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@296621 0039d316-1c4b-4281-b951-d872f2087c98

git_find_releases.py

@@ -19,7 +19,12 @@ import git_common as git
 
 
 def GetNameForCommit(sha1):
-  return re.sub(r'~.*$', '', git.run('name-rev', '--tags', '--name-only', sha1))
+  name = re.sub(r'~.*$', '', git.run('name-rev', '--tags', '--name-only', sha1))
+  if name == 'undefined':
+    name = git.run(
+        'name-rev', '--refs', 'remotes/branch-heads/*', '--name-only',
+        sha1) + ' [untagged]'
+  return name
 
 
 def GetMergesForCommit(sha1):