completion: add git config gc completions

Add missing completion for git config gc options:

* gc.aggressiveDepth
* gc.autoDetach
* gc.logExpiry
* gc.worktreePruneExpire

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -2464,15 +2464,19 @@ _git_config ()
 		format.thread
 		format.to
 		gc.
+		gc.aggressiveDepth
 		gc.aggressiveWindow
 		gc.auto
+		gc.autoDetach
 		gc.autopacklimit
+		gc.logExpiry
 		gc.packrefs
 		gc.pruneexpire
 		gc.reflogexpire
 		gc.reflogexpireunreachable
 		gc.rerereresolved
 		gc.rerereunresolved
+		gc.worktreePruneExpire
 		gitcvs.allbinary
 		gitcvs.commitmsgannotation
 		gitcvs.dbTableNamePrefix