completion: For consistency, change "git rev-parse" to __gitdir calls

Signed-off-by: Ted Pavlic <ted@tedpavlic.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -80,7 +80,7 @@ __gitdir ()
 # returns text to add to bash PS1 prompt (includes branch name)
 __git_ps1 ()
 {
-	local g="$(git rev-parse --git-dir 2>/dev/null)"
+	local g="$(__gitdir)"
 	if [ -n "$g" ]; then
 		local r
 		local b
@@ -1797,7 +1797,7 @@ _gitk ()
 	__git_has_doubledash && return
 
 	local cur="${COMP_WORDS[COMP_CWORD]}"
-	local g="$(git rev-parse --git-dir 2>/dev/null)"
+	local g="$(__gitdir)"
 	local merge=""
 	if [ -f $g/MERGE_HEAD ]; then
 		merge="--merge"