completion: don't guard git executions with __gitdir()

Three completion functions, namely __git_index_files(), __git_heads()
and __git_tags(), first run __gitdir() and check that the path it
outputs exists, i.e. that there is a git repository, and run a git
command only if there is one.

After the previous changes in this series there are no further uses of
__gitdir()'s output in these functions besides those checks.  And
those checks are unnecessary, because we can just execute those git
commands outside of a repository and let them error out.  We don't
perform such a check in other places either.

Remove this check and the __gitdir() call from these functions,
sparing the fork()+exec() overhead of the command substitution and the
potential 'git rev-parse' execution.

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

contrib/completion/git-completion.bash

@@ -312,9 +312,8 @@ __git_ls_files_helper ()
 #    slash.
 __git_index_files ()
 {
-	local dir="$(__gitdir)" root="${2-.}" file
+	local root="${2-.}" file
 
-	if [ -d "$dir" ]; then
 	__git_ls_files_helper "$root" "$1" |
 	while read -r file; do
 		case "$file" in
@@ -322,25 +321,16 @@ __git_index_files ()
 		*) echo "$file" ;;
 		esac
 	done | sort | uniq
-	fi
 }
 
 __git_heads ()
 {
-	local dir="$(__gitdir)"
-	if [ -d "$dir" ]; then
 	__git for-each-ref --format='%(refname:short)' refs/heads
-		return
-	fi
 }
 
 __git_tags ()
 {
-	local dir="$(__gitdir)"
-	if [ -d "$dir" ]; then
 	__git for-each-ref --format='%(refname:short)' refs/tags
-		return
-	fi
 }
 
 # Lists refs from the local (by default) or from a remote repository.