stash: honor stash.index in apply, pop modes

With stash.index=true, git-stash(1) command now tries to reinstate the
index by default in the "apply" and "pop" modes. Not doing so creates a
common trap [1], [2]: "git stash apply" is not the reverse of "git stash
push" because carefully staged indices are lost and have to be manually
recreated. OTOH, this mode is not always desirable and may create more
conflicts when applying stashes. As usual, "--no-index" will disable
this behavior if you set "stash.index".

[1]: https://lore.kernel.org/git/CAPx1GvcxyDDQmCssMjEnt6JoV6qPc5ZUpgPLX3mpUC_4PNYA1w@mail.gmail.com/
[2]: https://lore.kernel.org/git/c5a811ac-8cd3-c389-ac6d-29020a648c87@gmail.com/

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: benknoble

Documentation/config/stash.adoc

@@ -1,3 +1,8 @@
+stash.index::
+	If this is set to true, `git stash apply` and `git stash pop` will
+	behave as if `--index` was supplied. Defaults to false. See the
+	descriptions in linkgit:git-stash[1].
+
 stash.showIncludeUntracked::
 	If this is set to true, the `git stash show` command will show
 	the untracked files of a stash entry.  Defaults to false. See

builtin/stash.c

@@ -130,6 +130,7 @@ static struct strbuf stash_index_path = STRBUF_INIT;
 static int show_stat = 1;
 static int show_patch;
 static int show_include_untracked;
+static int use_index;
 
 /*
  * w_commit is set to the commit containing the working tree
@@ -662,7 +663,7 @@ static int apply_stash(int argc, const char **argv, const char *prefix,
 {
 	int ret = -1;
 	int quiet = 0;
-	int index = 0;
+	int index = use_index;
 	struct stash_info info = STASH_INFO_INIT;
 	struct option options[] = {
 		OPT__QUIET(&quiet, N_("be quiet, only report errors")),
@@ -759,7 +760,7 @@ static int pop_stash(int argc, const char **argv, const char *prefix,
 		     struct repository *repo UNUSED)
 {
 	int ret = -1;
-	int index = 0;
+	int index = use_index;
 	int quiet = 0;
 	struct stash_info info = STASH_INFO_INIT;
 	struct option options[] = {
@@ -864,6 +865,10 @@ static int git_stash_config(const char *var, const char *value,
 		show_include_untracked = git_config_bool(var, value);
 		return 0;
 	}
+	if (!strcmp(var, "stash.index")) {
+		use_index = git_config_bool(var, value);
+		return 0;
+	}
 	return git_diff_basic_config(var, value, ctx, cb);
 }
 

t/t3903-stash.sh

@@ -1595,4 +1595,41 @@ test_expect_success 'stash apply reports a locked index' '
 	)
 '
 
+test_expect_success 'stash.index=true implies --index' '
+	# setup for a few related tests
+	test_commit file base &&
+	echo index >file &&
+	git add file &&
+	echo working >file &&
+	git stash &&
+
+	test_when_finished "git reset --hard" &&
+	git -c stash.index=true stash apply &&
+	echo index >expect &&
+	git show :0:file >actual &&
+	test_cmp expect actual &&
+	echo working >expect &&
+	test_cmp expect file
+'
+
+test_expect_success 'stash.index=true overridden by --no-index' '
+	test_when_finished "git reset --hard" &&
+	git -c stash.index=true stash apply --no-index &&
+	echo base >expect &&
+	git show :0:file >actual &&
+	test_cmp expect actual &&
+	echo working >expect &&
+	test_cmp expect file
+'
+
+test_expect_success 'stash.index=false overridden by --index' '
+	test_when_finished "git reset --hard" &&
+	git -c stash.index=false stash apply --index &&
+	echo index >expect &&
+	git show :0:file >actual &&
+	test_cmp expect actual &&
+	echo working >expect &&
+	test_cmp expect file
+'
+
 test_done