Make correction to grep pattern to grab git hash for given commit position.

Note that the pattern used to check commit position appears two times in reverted
CL, therefore match the pattern at begining of the line

BUG=413365,412054
NOTRY=true

Review URL: https://codereview.chromium.org/564873003

Cr-Commit-Position: refs/heads/master@{#294463}

tools/auto_bisect/source_control.py

@@ -154,7 +154,7 @@ class GitSourceControl(SourceControl):
 
       for i in search_range:
         svn_pattern = 'git-svn-id: %s@%d' % (depot_svn, i)
-        commit_position_pattern = 'Cr-Commit-Position: .*@{#%d}' % i
+        commit_position_pattern = '^Cr-Commit-Position: .*@{#%d}' % i
         cmd = ['log', '--format=%H', '-1', '--grep', svn_pattern,
                '--grep', commit_position_pattern, 'origin/master']