scalar: teach 'clone' to support the --single-branch option

Just like `git clone`, the `scalar clone` command now also offers to
restrict the clone to a single branch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/scalar/scalar.c

@@ -327,12 +327,15 @@ static char *remote_default_branch(const char *url)
 static int cmd_clone(int argc, const char **argv)
 {
 	const char *branch = NULL;
-	int full_clone = 0;
+	int full_clone = 0, single_branch = 0;
 	struct option clone_options[] = {
 		OPT_STRING('b', "branch", &branch, N_("<branch>"),
 			   N_("branch to checkout after clone")),
 		OPT_BOOL(0, "full-clone", &full_clone,
 			 N_("when cloning, create full working directory")),
+		OPT_BOOL(0, "single-branch", &single_branch,
+			 N_("only download metadata for the branch that will "
+			    "be checked out")),
 		OPT_END(),
 	};
 	const char * const clone_usage[] = {
@@ -403,7 +406,9 @@ static int cmd_clone(int argc, const char **argv)
 
 	if (set_config("remote.origin.url=%s", url) ||
 	    set_config("remote.origin.fetch="
-		       "+refs/heads/*:refs/remotes/origin/*") ||
+		       "+refs/heads/%s:refs/remotes/origin/%s",
+		       single_branch ? branch : "*",
+		       single_branch ? branch : "*") ||
 	    set_config("remote.origin.promisor=true") ||
 	    set_config("remote.origin.partialCloneFilter=blob:none")) {
 		res = error(_("could not configure remote in '%s'"), dir);

contrib/scalar/scalar.txt

@@ -8,7 +8,7 @@ scalar - an opinionated repository management tool
 SYNOPSIS
 --------
 [verse]
-scalar clone [--branch <main-branch>] [--full-clone] <url> [<enlistment>]
+scalar clone [--single-branch] [--branch <main-branch>] [--full-clone] <url> [<enlistment>]
 scalar list
 scalar register [<enlistment>]
 scalar unregister [<enlistment>]
@@ -57,6 +57,16 @@ HEAD[:<directory>]`.
 	Instead of checking out the branch pointed to by the cloned
 	repository's HEAD, check out the `<name>` branch instead.
 
+--[no-]single-branch::
+	Clone only the history leading to the tip of a single branch, either
+	specified by the `--branch` option or the primary branch remote's
+	`HEAD` points at.
++
+Further fetches into the resulting repository will only update the
+remote-tracking branch for the branch this option was used for the initial
+cloning. If the HEAD at the remote did not point at any branch when
+`--single-branch` clone was made, no remote-tracking branch is created.
+
 --[no-]full-clone::
 	A sparse-checkout is initialized by default. This behavior can be
 	turned off via `--full-clone`.

contrib/scalar/t/t9099-scalar.sh

@@ -45,13 +45,17 @@ test_expect_success 'set up repository to clone' '
 
 test_expect_success 'scalar clone' '
 	second=$(git rev-parse --verify second:second.t) &&
-	scalar clone "file://$(pwd)" cloned &&
+	scalar clone "file://$(pwd)" cloned --single-branch &&
 	(
 		cd cloned/src &&
 
 		git config --get --global --fixed-value maintenance.repo \
 			"$(pwd)" &&
 
+		git for-each-ref --format="%(refname)" refs/remotes/origin/ >actual &&
+		echo "refs/remotes/origin/parallel" >expect &&
+		test_cmp expect actual &&
+
 		test_path_is_missing 1/2 &&
 		test_must_fail git rev-list --missing=print $second &&
 		git rev-list $second &&