checkout: introduce checkout.overlayMode config

In the previous patch we introduced a new no-overlay mode for git
checkout.  Some users (such as the author of this commit) may want to
have this mode turned on by default as it matches their mental model
more closely.  Make that possible by introducing a new config option
to that extend.
Signed-off-by: NThomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config/checkout.txt

@@ -21,3 +21,10 @@ checkout.optimizeNewBranch::
 	will not update the skip-worktree bit in the index nor add/remove
 	files in the working directory to reflect the current sparse checkout
 	settings nor will it show the local changes.
+
+checkout.overlayMode::
+	In the default overlay mode, `git checkout` never
+	removes files from the index or the working tree.  When
+	setting `checkout.overlayMode` to false, files that appear in
+	the index and working tree, but not in <tree-ish> are removed,
+	to make them match <tree-ish> exactly.

builtin/checkout.c

@@ -1019,13 +1019,19 @@ static int switch_branches(const struct checkout_opts *opts,
 
 static int git_checkout_config(const char *var, const char *value, void *cb)
 {
+	struct checkout_opts *opts = cb;
+
 	if (!strcmp(var, "checkout.optimizenewbranch")) {
 		checkout_optimize_new_branch = git_config_bool(var, value);
 		return 0;
 	}
 
+	if (!strcmp(var, "checkout.overlaymode")) {
+		opts->overlay_mode = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "diff.ignoresubmodules")) {
-		struct checkout_opts *opts = cb;
 		handle_ignore_submodules_arg(&opts->diff_options, value);
 		return 0;
 	}

t/t2025-checkout-no-overlay.sh

@@ -44,4 +44,14 @@ test_expect_success '--no-overlay --theirs with D/F conflict deletes file' '
 	test_path_is_missing file1
 '
 
+test_expect_success 'checkout with checkout.overlayMode=false deletes files not in <tree-ish>' '
+	>file &&
+	mkdir dir &&
+	>dir/file1 &&
+	git add file dir/file1 &&
+	git -c checkout.overlayMode=false checkout HEAD -- file &&
+	test_path_is_missing file &&
+	test_path_is_file dir/file1
+'
+
 test_done