repo-pull: Allow to ignore local branch name

This commit allows users to omit `--branch` from command line options.
`repo_pull.py` will not call `repo start` if `--branch` is unspecified.

Test: repo_pull.py bash -g GERRIT QUERY
Change-Id: I62380485f057bd35ab8ae4d5e70372ff5d05fcc5
2 files changed