doc: move git-rev-parse from porcelain to plumbing

git-rev-parse mostly seems like plumbing, and is more usd in
scripts than in regular use. Online it's often mentioned as
a plumbing command. Nonetheless it's listed under porcelain
interrogators in `man git`. It seems appropriate to formally
move git-rev-parse to plumbing interrogators.

Signed-off-by: Daniels Umanovskis <daniels@umanovskis.se>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

command-list.txt

@@ -152,7 +152,7 @@ git-rerere                              ancillaryinterrogators
 git-reset                               mainporcelain           worktree
 git-revert                              mainporcelain
 git-rev-list                            plumbinginterrogators
-git-rev-parse                           ancillaryinterrogators
+git-rev-parse                           plumbinginterrogators
 git-rm                                  mainporcelain           worktree
 git-send-email                          foreignscminterface             complete
 git-send-pack                           synchingrepositories