repo-pull: Add checkout for merge commits

This commit adds `-m checkout` for merge commits.  If `-m checkout` is
specified, `repo_pull.py` will pull the merge commits with
`git checkout FETCH_HEAD`.

Test: repo_pull.py bash -g GERRIT QUERY -m checkout -b local_branch_name
Change-Id: I6cbedbd2941c63c8bb0591b6033a136829ffda47