Tighten GITHUB_BRANCH_PATTERN to reject malformed refs

Repository names could previously contain a slash and branches could
be empty, which produced bogus GitHubBranch values instead of a clear
parse error. Addresses CodeRabbit review comment on PR #71.

Author: RadekManak

rebasebot/github.py

@@ -22,7 +22,7 @@
 from github import Auth, Github, GithubIntegration, UnknownObjectException
 
 logger = logging.getLogger()
-GITHUB_BRANCH_PATTERN = re.compile(r"^(?P<organization>[^/]+)/(?P<name>[^:]+):(?P<branch>.*)$")
+GITHUB_BRANCH_PATTERN = re.compile(r"^(?P<organization>[^/:]+)/(?P<name>[^/:]+):(?P<branch>[^:]+)$")
 
 
 @dataclass

tests/test_cli.py

@@ -101,6 +101,8 @@ def test_github_branch_parse_valid(self, get_valid_cli_args, arg, github_ref, ex
             "/kubernetes/autoscaler:master",
             "fooo",
             "asdasdasdqwe/asdasd\\asdsadasd",
+            "kubernetes/auto/scaler:master",
+            "kubernetes/autoscaler:",
         ),
     )
     @pytest.mark.parametrize("arg", ["source", "dest", "rebase"])