revparse: support open-ended ranges

Support '..' and '...' ranges where one side is not specified.
The unspecified side defaults to HEAD.

Closes #4223

src/revparse.c

@@ -892,6 +892,17 @@ int git_revparse(
 		const char *rstr;
 		revspec->flags = GIT_REVPARSE_RANGE;
 
+		/*
+		 * Following git.git, don't allow '..' because it makes command line
+		 * arguments which can be either paths or revisions ambiguous when the
+		 * path is almost certainly intended. The empty range '...' is still
+		 * allowed.
+		 */
+		if (!git__strcmp(spec, "..")) {
+			giterr_set(GITERR_INVALID, "Invalid pattern '..'");
+			return GIT_EINVALIDSPEC;
+		}
+
 		lstr = git__substrdup(spec, dotdot - spec);
 		rstr = dotdot + 2;
 		if (dotdot[2] == '.') {
@@ -899,9 +910,17 @@ int git_revparse(
 			rstr++;
 		}
 
-		error = git_revparse_single(&revspec->from, repo, lstr);
-		if (!error)
-			error = git_revparse_single(&revspec->to, repo, rstr);
+		error = git_revparse_single(
+			&revspec->from,
+			repo,
+			*lstr == '\0' ? "HEAD" : lstr);
+
+		if (!error) {
+			error = git_revparse_single(
+				&revspec->to,
+				repo,
+				*rstr == '\0' ? "HEAD" : rstr);
+		}
 
 		git__free((void*)lstr);
 	} else {

tests/refs/revparse.c

@@ -122,6 +122,14 @@ static void test_id(
 	test_id_inrepo(spec, expected_left, expected_right, expected_flags, g_repo);
 }
 
+static void test_invalid_revspec(const char* invalid_spec)
+{
+	git_revspec revspec;
+
+	cl_assert_equal_i(
+		GIT_EINVALIDSPEC, git_revparse(&revspec, g_repo, invalid_spec));
+}
+
 void test_refs_revparse__initialize(void)
 {
 	cl_git_pass(git_repository_open(&g_repo, cl_fixture("testrepo.git")));
@@ -749,6 +757,33 @@ void test_refs_revparse__parses_range_operator(void)
 		"4a202b346bb0fb0db7eff3cffeb3c70babbd2045",
 		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
 		GIT_REVPARSE_RANGE | GIT_REVPARSE_MERGE_BASE);
+
+	test_id("HEAD~3..",
+		"4a202b346bb0fb0db7eff3cffeb3c70babbd2045",
+		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
+		GIT_REVPARSE_RANGE);
+
+	test_id("HEAD~3...",
+		"4a202b346bb0fb0db7eff3cffeb3c70babbd2045",
+		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
+		GIT_REVPARSE_RANGE | GIT_REVPARSE_MERGE_BASE);
+
+	test_id("..HEAD~3",
+		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
+		"4a202b346bb0fb0db7eff3cffeb3c70babbd2045",
+		GIT_REVPARSE_RANGE);
+
+	test_id("...HEAD~3",
+		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
+		"4a202b346bb0fb0db7eff3cffeb3c70babbd2045",
+		GIT_REVPARSE_RANGE | GIT_REVPARSE_MERGE_BASE);
+
+	test_id("...",
+		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
+		"a65fedf39aefe402d3bb6e24df4d4f5fe4547750",
+		GIT_REVPARSE_RANGE | GIT_REVPARSE_MERGE_BASE);
+
+	test_invalid_revspec("..");
 }
 
 void test_refs_revparse__ext_retrieves_both_the_reference_and_its_target(void)