repo-pull: Add `--pick` for non-merge commits

This commit adds the `--pick` command line option so that users can
choose the git command for the non-merge commits.

Test: repo_pull.py pull -g GERRIT QUERY -p merge-ff-only
Change-Id: Ib8bdaa1418dc2fb48d0f0e3c4eb995a23c007563
2 files changed