doc: move git-get-tar-commit-id to plumbing

This is definitely a low-level command, it's hard to argue
against it belonging in plumbing.

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

Author: umanovskis

command-list.txt

@@ -96,7 +96,7 @@ git-for-each-ref                        plumbinginterrogators
 git-format-patch                        mainporcelain
 git-fsck                                ancillaryinterrogators          complete
 git-gc                                  mainporcelain
-git-get-tar-commit-id                   ancillaryinterrogators
+git-get-tar-commit-id                   plumbinginterrogators
 git-grep                                mainporcelain           info
 git-gui                                 mainporcelain
 git-hash-object                         plumbingmanipulators