Rebase explicitly origin/master instead of guessing from the remote.

Not sure this code worked.

R=cmp@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/10913193

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@156059 0039d316-1c4b-4281-b951-d872f2087c98

update_depot_tools.bat

@@ -36,7 +36,7 @@ cd /d "%~dp0."
 call git config remote.origin.fetch > NUL
 if errorlevel 1 goto :GIT_SVN_UPDATE
 call git fetch -q origin > NUL
-call git rebase -q origin > NUL
+call git rebase -q origin/master > NUL
 goto :EOF
 
 :GIT_SVN_UPDATE