Merge branch 'rf/completion'

Completion updates.

* rf/completion:
completion: add git config credentialCache.ignoreSIGHUP
completion: add git config credential completions
completion: add git config advice completions
completion: add git config am.threeWay completion
completion: add git config core completions
completion: add git config gc completions

diff --cc contrib/completion/git-completion.bash
Simple merge