push: revparse refspec source, so you can push things that are not refs

I want to push a commit by OID to a remote branch. Currently, push parses the refspecs such that the source must be the name of a ref (it uses git_reference_name_to_id to resolve it). This commit changes it so push uses git_revparse_single to resolve the source of the refspec. This allows for OIDs or other revs (e.g. `HEAD~2`) to be pushed.

src/libgit2/push.c

@@ -385,11 +385,18 @@ static int calculate_work(git_push *push)
 	git_vector_foreach(&push->specs, i, spec) {
 		if (spec->refspec.src && spec->refspec.src[0]!= '\0') {
 			/* This is a create or update.  Local ref must exist. */
-			if (git_reference_name_to_id(
-					&spec->loid, push->repo, spec->refspec.src) < 0) {
-				git_error_set(GIT_ERROR_REFERENCE, "no such reference '%s'", spec->refspec.src);
+
+			git_object *obj;
+			int error = git_revparse_single(&obj, push->repo, spec->refspec.src);
+
+			if (error < 0) {
+				git_object_free(obj);
+				git_error_set(GIT_ERROR_REFERENCE, "src refspec %s does not match any", spec->refspec.src);
 				return -1;
 			}
+
+			git_oid_cpy(git_object_id(obj), &spec->loid);
+			git_object_free(obj);
 		}
 
 		/* Remote ref may or may not (e.g. during create) already exist. */

tests/libgit2/online/push.c

@@ -841,13 +841,28 @@ void test_online_push__bad_refspecs(void)
 
 void test_online_push__expressions(void)
 {
-	/* TODO: Expressions in refspecs doesn't actually work yet */
-	const char *specs_left_expr[] = { "refs/heads/b2~1:refs/heads/b2" };
+	const char *specs_left_expr[] = {
+		"refs/heads/b3~1:refs/heads/b2",
+		"b4:refs/heads/b4",
+		"fa38b91f199934685819bea316186d8b008c52a2:refs/heads/b5",
+		"951bbbb:refs/heads/b6"
+	};
+	push_status exp_stats[] = {
+		{ "refs/heads/b2", 1 },
+		{ "refs/heads/b4", 1 },
+		{ "refs/heads/b5", 1 },
+		{ "refs/heads/b6", 1 }
+	};
+	expected_ref exp_refs[] = {
+		{ "refs/heads/b2", &_oid_b2 },
+		{ "refs/heads/b4", &_oid_b4 },
+		{ "refs/heads/b5", &_oid_b5 },
+		{ "refs/heads/b6", &_oid_b6 }
+	};
 
-	/* TODO: Find a more precise way of checking errors than a exit code of -1. */
 	do_push(specs_left_expr, ARRAY_SIZE(specs_left_expr),
-		NULL, 0,
-		NULL, 0, -1, 0, 0);
+		exp_stats, ARRAY_SIZE(exp_stats),
+		exp_refs, ARRAY_SIZE(exp_refs), 0, 1, 1);
 }
 
 void test_online_push__notes(void)