Merge branch 'mm/credential-plumbing'

Expose the credential API to scripted Porcelain writers.

* mm/credential-plumbing:
git-remote-mediawiki: update comments to reflect credential support
git-remote-mediawiki: add credential support
git credential fill: output the whole 'struct credential'
add 'git credential' plumbing command

diff --cc Makefile
Simple merge

diff --cc builtin.h
Simple merge