Fix git_footers to accept refs with periods in them.

For example, v8 has refs/heads/1.4.2

TBR=machenbach@chromium.org
BUG=chromium:674448

Change-Id: Ia3405ee402a01153fc9744457d8fee9385446cae
Reviewed-on: https://chromium-review.googlesource.com/420285Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

git_footers.py

@@ -14,7 +14,7 @@ import git_common as git
 
 
 FOOTER_PATTERN = re.compile(r'^\s*([\w-]+): (.*)$')
-CHROME_COMMIT_POSITION_PATTERN = re.compile(r'^([\w/-]+)@{#(\d+)}$')
+CHROME_COMMIT_POSITION_PATTERN = re.compile(r'^([\w/\-\.]+)@{#(\d+)}$')
 GIT_SVN_ID_PATTERN = re.compile('^([^\s@]+)@(\d+)')