Do not assign revision when checking the commit existence.

'git rev-parse <revision> will return same revision result with
newline appended if revision exists. The newline will cause the
following checkout command will fail, so remove the assignment.

BUG=
R=agable@chromium.org, rogerta@chromium.org

Review URL: https://codereview.chromium.org/317783008

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@277615 0039d316-1c4b-4281-b951-d872f2087c98

checkout.py

@@ -594,11 +594,11 @@ class GitCheckout(CheckoutBase):
       try:
         # Look if the commit hash already exist. If so, we can skip a
         # 'git fetch' call.
-        revision = self._check_output_git(['rev-parse', revision])
+        revision = self._check_output_git(['rev-parse', revision]).rstrip()
       except subprocess.CalledProcessError:
         self._check_call_git(
             ['fetch', self.remote, self.remote_branch, '--quiet'])
-        revision = self._check_output_git(['rev-parse', revision])
+        revision = self._check_output_git(['rev-parse', revision]).rstrip()
       self._check_call_git(['checkout', '--force', '--quiet', revision])
     else:
       branches, active = self._branches()