rev-list --min-parents,--max-parents: doc, test and completion

This also adds test for "--merges" and "--no-merges" which we did not
have so far.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1572,6 +1572,8 @@ __git_log_common_options="
 	--max-count=
 	--max-age= --since= --after=
 	--min-age= --until= --before=
+	--min-parents= --max-parents=
+	--no-min-parents --no-max-parents
 "
 # Options that go well for log and gitk (not shortlog)
 __git_log_gitk_options="