completion: extract repository discovery from __gitdir()

To prepare for caching the path to the repository in the following
commit, extract the repository discovering part of __gitdir() into the
__git_find_repo_path() helper function, which stores the found path in
the $__git_repo_path variable instead of printing it.  Make __gitdir()
a wrapper around this new function.  Declare $__git_repo_path local in
the toplevel completion functions __git_main() and __gitk_main() to
ensure that it never leaks into the environment and influences
subsequent completions (though this isn't necessary right now, as
__gitdir() is still only executed in subshells, but will matter for
the following commit).

Adjust tests checking __gitdir() or any other completion function
calling __gitdir() to perform those checks in a subshell to prevent
$__git_repo_path from leaking into the test environment.  Otherwise
leave the tests unchanged to demonstrate that this change doesn't
alter __gitdir()'s behavior.

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

contrib/completion/git-completion.bash

@@ -34,26 +34,35 @@ case "$COMP_WORDBREAKS" in
 *)   COMP_WORDBREAKS="$COMP_WORDBREAKS:"
 esac
 
+# Discovers the path to the git repository taking any '--git-dir=<path>' and
+# '-C <path>' options into account and stores it in the $__git_repo_path
+# variable.
+__git_find_repo_path ()
+{
+	if [ -n "${__git_C_args-}" ]; then
+		__git_repo_path="$(git "${__git_C_args[@]}" \
+			${__git_dir:+--git-dir="$__git_dir"} \
+			rev-parse --absolute-git-dir 2>/dev/null)"
+	elif [ -n "${__git_dir-}" ]; then
+		test -d "$__git_dir" &&
+		__git_repo_path="$__git_dir"
+	elif [ -n "${GIT_DIR-}" ]; then
+		test -d "${GIT_DIR-}" &&
+		__git_repo_path="$GIT_DIR"
+	elif [ -d .git ]; then
+		__git_repo_path=.git
+	else
+		__git_repo_path="$(git rev-parse --git-dir 2>/dev/null)"
+	fi
+}
+
 # __gitdir accepts 0 or 1 arguments (i.e., location)
 # returns location of .git repo
 __gitdir ()
 {
 	if [ -z "${1-}" ]; then
-		if [ -n "${__git_C_args-}" ]; then
+		__git_find_repo_path || return 1
-			git "${__git_C_args[@]}" \
+		echo "$__git_repo_path"
-				${__git_dir:+--git-dir="$__git_dir"} \
-				rev-parse --absolute-git-dir 2>/dev/null
-		elif [ -n "${__git_dir-}" ]; then
-			test -d "$__git_dir" || return 1
-			echo "$__git_dir"
-		elif [ -n "${GIT_DIR-}" ]; then
-			test -d "${GIT_DIR-}" || return 1
-			echo "$GIT_DIR"
-		elif [ -d .git ]; then
-			echo .git
-		else
-			git rev-parse --git-dir 2>/dev/null
-		fi
 	elif [ -d "$1/.git" ]; then
 		echo "$1/.git"
 	else
@@ -2783,7 +2792,7 @@ _git_worktree ()
 
 __git_main ()
 {
-	local i c=1 command __git_dir
+	local i c=1 command __git_dir __git_repo_path
 	local __git_C_args C_args_count=0
 
 	while [ $c -lt $cword ]; do
@@ -2855,6 +2864,7 @@ __gitk_main ()
 {
 	__git_has_doubledash && return
 
+	local __git_repo_path
 	local g="$(__gitdir)"
 	local merge=""
 	if [ -f "$g/MERGE_HEAD" ]; then

t/t9902-completion.sh

@@ -147,19 +147,25 @@ test_expect_success '__gitdir - from command line (through $__git_dir)' '
 
 test_expect_success '__gitdir - repo as argument' '
 	echo "otherrepo/.git" >expected &&
-	__gitdir "otherrepo" >"$actual" &&
+	(
+		__gitdir "otherrepo" >"$actual"
+	) &&
 	test_cmp expected "$actual"
 '
 
 test_expect_success '__gitdir - remote as argument' '
 	echo "remote" >expected &&
-	__gitdir "remote" >"$actual" &&
+	(
+		__gitdir "remote" >"$actual"
+	) &&
 	test_cmp expected "$actual"
 '
 
 test_expect_success '__gitdir - .git directory in cwd' '
 	echo ".git" >expected &&
-	__gitdir >"$actual" &&
+	(
+		__gitdir >"$actual"
+	) &&
 	test_cmp expected "$actual"
 '
 
@@ -450,7 +456,9 @@ test_expect_success '__git_remotes - list remotes from $GIT_DIR/remotes and from
 	git remote add remote_in_config_1 git://remote_1 &&
 	test_when_finished "git remote remove remote_in_config_2" &&
 	git remote add remote_in_config_2 git://remote_2 &&
-	__git_remotes >actual &&
+	(
+		__git_remotes >actual
+	) &&
 	test_cmp expect actual
 '
 
@@ -459,8 +467,10 @@ test_expect_success '__git_is_configured_remote' '
 	git remote add remote_1 git://remote_1 &&
 	test_when_finished "git remote remove remote_2" &&
 	git remote add remote_2 git://remote_2 &&
+	(
 		verbose __git_is_configured_remote remote_2 &&
 		test_must_fail __git_is_configured_remote non-existent
+	)
 '
 
 test_expect_success 'setup for ref completion' '