reset: optionally setup worktree and refresh index on --mixed

Refreshing index requires work tree.  So we have two options: always
set up work tree (and refuse to reset if failing to do so), or make
refreshing index optional.

As refreshing index is not the main task, it makes more sense to make
it optional. This allows us to still work in a bare repository to update
what is in the index.

Reported-by: Patrick Palka <patrick@parcs.ath.cx>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -320,7 +320,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (reset_type == NONE)
 		reset_type = MIXED; /* by default */
 
-	if (reset_type != SOFT && reset_type != MIXED)
+	if (reset_type != SOFT && (reset_type != MIXED || get_git_work_tree()))
 		setup_work_tree();
 
 	if (reset_type == MIXED && is_bare_repository())
@@ -340,8 +340,9 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 			int flags = quiet ? REFRESH_QUIET : REFRESH_IN_PORCELAIN;
 			if (read_from_tree(&pathspec, sha1))
 				return 1;
-			refresh_index(&the_index, flags, NULL, NULL,
-				      _("Unstaged changes after reset:"));
+			if (get_git_work_tree())
+				refresh_index(&the_index, flags, NULL, NULL,
+					      _("Unstaged changes after reset:"));
 		} else {
 			int err = reset_index(sha1, reset_type, quiet);
 			if (reset_type == KEEP && !err)

t/t7102-reset.sh

@@ -535,4 +535,15 @@ test_expect_success 'reset with paths accepts tree' '
 	git diff HEAD --exit-code
 '
 
+test_expect_success 'reset --mixed sets up work tree' '
+	git init mixed_worktree &&
+	(
+		cd mixed_worktree &&
+		test_commit dummy
+	) &&
+	: >expect &&
+	git --git-dir=mixed_worktree/.git --work-tree=mixed_worktree reset >actual &&
+	test_cmp expect actual
+'
+
 test_done