commit: add a commit.verbose config variable

commit: add a commit.verbose config variable

Add commit.verbose configuration variable as a convenience for those
who always prefer --verbose.

Add tests to check the behavior introduced by this commit and also to
verify that behavior of status doesn't break because of this commit.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>