builtin-commit.c: show on which branch a commit was added

builtin-commit.c: show on which branch a commit was added

This outputs the current branch on which a commit was created, just for
reference. For example:

	Created commit 6d42875 on master: Fix submodule invalid command error

This also reminds the committer when he is on a detached HEAD:

	Created commit 02a7172 on detached HEAD: Also do this for 'git commit --amend'

Signed-off-by: Pieter de Bie <pdebie@ai.rug.nl>
Signed-off-by: Junio C Hamano <gitster@pobox.com>