Merge branch 'es/check-mailmap'

A new command to allow scripts to query the mailmap information.

* es/check-mailmap:
  t4203: test check-mailmap command invocation
  builtin: add git-check-mailmap command

diff --cc .gitignore
Simple merge

diff --cc Makefile
Simple merge

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

diff --cc git.c
Simple merge

diff --cc t/t4203-mailmap.sh
Simple merge