clone: allow "--dissociate" without reference

The "--reference" option is not the only way to provide a repository
to borrow objects from.  A repository that borrows from another
repository can be cloned with "clone --local" and the resulting
repository will borrow from the same repository, which the user
may want to "--dissociate" from.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -104,8 +104,13 @@ objects from the source repository into a pack in the cloned repository.
 --dissociate::
 	Borrow the objects from reference repositories specified
 	with the `--reference` options only to reduce network
-	transfer and stop borrowing from them after a clone is made
-	by making necessary local copies of borrowed objects.
+	transfer, and stop borrowing from them after a clone is made
+	by making necessary local copies of borrowed objects.  This
+	option can also be used when cloning locally from a
+	repository that already borrows objects from another
+	repository--the new repository will borrow objects from the
+	same repository, and this option can be used to stop the
+	borrowing.
 
 --quiet::
 -q::

builtin/clone.c

@@ -801,11 +801,15 @@ static void write_refspec_config(const char *src_ref_prefix,
 static void dissociate_from_references(void)
 {
 	static const char* argv[] = { "repack", "-a", "-d", NULL };
+	char *alternates = git_pathdup("objects/info/alternates");
 
-	if (run_command_v_opt(argv, RUN_GIT_CMD|RUN_COMMAND_NO_STDIN))
-		die(_("cannot repack to clean up"));
-	if (unlink(git_path("objects/info/alternates")) && errno != ENOENT)
-		die_errno(_("cannot unlink temporary alternates file"));
+	if (!access(alternates, F_OK)) {
+		if (run_command_v_opt(argv, RUN_GIT_CMD|RUN_COMMAND_NO_STDIN))
+			die(_("cannot repack to clean up"));
+		if (unlink(alternates) && errno != ENOENT)
+			die_errno(_("cannot unlink temporary alternates file"));
+	}
+	free(alternates);
 }
 
 int cmd_clone(int argc, const char **argv, const char *prefix)
@@ -954,10 +958,6 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 
 	if (option_reference.nr)
 		setup_reference();
-	else if (option_dissociate) {
-		warning(_("--dissociate given, but there is no --reference"));
-		option_dissociate = 0;
-	}
 
 	fetch_pattern = value.buf;
 	refspec = parse_fetch_refspec(1, &fetch_pattern);

t/t5700-clone-reference.sh

@@ -210,4 +210,15 @@ test_expect_success 'clone, dissociate from partial reference and repack' '
 	test_line_count = 1 packs.txt
 '
 
+test_expect_success 'clone, dissociate from alternates' '
+	rm -fr A B C &&
+	test_create_repo A &&
+	commit_in A file1 &&
+	git clone --reference=A A B &&
+	test_line_count = 1 B/.git/objects/info/alternates &&
+	git clone --local --dissociate B C &&
+	! test -f C/.git/objects/info/alternates &&
+	( cd C && git fsck )
+'
+
 test_done