`git cl split`: Include dirname as part of branch names

This CL changes the way `git cl split` generates branch names during
upload. Specifically, it attempts to derive a common directory for all
the files in each CL, and includes that dirname as part of the branch
name, in addition to a hash of the files in the CL. This makes it a bit
easier to guess what's in a branch, given its name.

Since the hash is still included, branch names should remain unique.
Finding a common directory should be deterministic, so we can still rely
on branches having the same name if the script is re-run with the same
splitting.

Bug: 389069356
Change-Id: I70490258755f13962f3db5c835619caa24176af9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6243377
Commit-Queue: Devon Loehr <dloehr@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>

split_cl.py

@@ -99,14 +99,35 @@ def EnsureInGitRepository():
     git.run('rev-parse')
 
 
-def CreateBranchForOneCL(prefix, files, upstream):
+def CreateBranchName(prefix: str, files: List[Tuple[str, str]]) -> str:
+    """
+    Given a sub-CL as a list of (action, file) pairs, create a unique and
+    deterministic branch name for it.
+    The name has the format <prefix>_<dirname>_<hash(files)>_split.
+    """
+    file_names = [file for _, file in files]
+    if len(file_names) == 1:
+        # Only one file, just use its directory as the common path
+        common_path = os.path.dirname(file_names[0])
+    else:
+        common_path = os.path.commonpath(file_names)
+    if not common_path:
+        # Files have nothing in common at all. Unlikely but possible.
+        common_path = "None"
+    # Replace path delimiter with underscore in common_path.
+    common_path = common_path.replace(os.path.sep, '_')
+    return f"{prefix}_{HashList(files):020}_{common_path}_split"
+
+
+def CreateBranchForOneCL(prefix: str, files: List[Tuple[str, str]],
+                         upstream: str) -> bool:
     """Creates a branch named |prefix| + "_" + |hash(files)| + "_split".
 
     Return false if the branch already exists. |upstream| is used as upstream
     for the created branch.
     """
     existing_branches = set(git.branches(use_limit=False))
-    branch_name = f'{prefix}_{HashList(files)}_split'
+    branch_name = CreateBranchName(prefix, files)
     if branch_name in existing_branches:
         return False
     git.run('checkout', '-t', upstream, '-b', branch_name)

tests/split_cl_test.py

@@ -143,7 +143,7 @@ class SplitClTest(unittest.TestCase):
         self.assertEqual(mock_git_run.call_count, 4)
         mock_git_run.assert_has_calls([
             mock.call("checkout", "-t", "upstream_branch", "-b",
-                      f"branch_to_upload_{split_cl.HashList(files)}_split"),
+                      split_cl.CreateBranchName("branch_to_upload", files)),
             mock.call("rm", os.path.join(abs_repository_path, "foo", "b.cc")),
             mock.call("checkout", "branch_to_upload", "--",
                       os.path.join(abs_repository_path, "bar", "a.cc")),
@@ -168,7 +168,8 @@ class SplitClTest(unittest.TestCase):
         reviewers = {"reviewer1@gmail.com"}
         mock_cmd_upload = mock.Mock()
         upload_cl_tester.mock_git_branches.return_value = [
-            "branch0", f"branch_to_upload_{split_cl.HashList(files)}_split"
+            "branch0",
+            split_cl.CreateBranchName("branch_to_upload", files)
         ]
         upload_cl_tester.DoUploadCl(directories, files, reviewers,
                                     mock_cmd_upload)