completion: fix most spots not respecting 'git --git-dir=<path>'

The completion script already respects the path to the repository
specified on the command line most of the time, here we add the
necessary '--git-dir=$(__gitdir)' options to most of the places where
git was executed without it.  The exceptions where said option is not
added are the git invocations:

  - in __git_refs() which are non-trivial and will be the subject of
    the following patch,

  - getting the list of git commands, merge strategies and archive
    formats, because these are independent from the repository and
    thus don't need it, and

  - the 'git rev-parse --git-dir' in __gitdir() itself.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -283,11 +283,13 @@ __gitcomp_file ()
 # argument, and using the options specified in the second argument.
 __git_ls_files_helper ()
 {
+	local dir="$(__gitdir)"
+
 	if [ "$2" == "--committable" ]; then
-		git -C "$1" diff-index --name-only --relative HEAD
+		git --git-dir="$dir" -C "$1" diff-index --name-only --relative HEAD
 	else
 		# NOTE: $2 is not quoted in order to support multiple options
-		git -C "$1" ls-files --exclude-standard $2
+		git --git-dir="$dir" -C "$1" ls-files --exclude-standard $2
 	fi 2>/dev/null
 }
 
@@ -408,7 +410,7 @@ __git_refs2 ()
 __git_refs_remotes ()
 {
 	local i hash
-	git ls-remote "$1" 'refs/heads/*' 2>/dev/null | \
+	git --git-dir="$(__gitdir)" ls-remote "$1" 'refs/heads/*' 2>/dev/null | \
 	while read -r hash i; do
 		echo "$i:refs/remotes/$1/${i#refs/heads/}"
 	done
@@ -1186,7 +1188,7 @@ _git_commit ()
 		return
 	esac
 
-	if git rev-parse --verify --quiet HEAD >/dev/null; then
+	if git --git-dir="$(__gitdir)" rev-parse --verify --quiet HEAD >/dev/null; then
 		__git_complete_index_file "--committable"
 	else
 		# This is the first commit
@@ -1486,7 +1488,7 @@ _git_log ()
 {
 	__git_has_doubledash && return
 
-	local g="$(git rev-parse --git-dir 2>/dev/null)"
+	local g="$(__gitdir)"
 	local merge=""
 	if [ -f "$g/MERGE_HEAD" ]; then
 		merge="--merge"