Print top reviewers for git cl split

Change-Id: I958ebe18327c73be2d2273eb5c230a6e6fba2d73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4686590
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Commit-Queue: Anne Redulla <aredulla@google.com>

split_cl.py

@@ -27,6 +27,11 @@ import git_common as git
 # in the past.
 CL_SPLIT_FORCE_LIMIT = 10
 
+# The maximum number of top reviewers to list. `git cl split` may send many CLs
+# to a single reviewer, so the top reviewers with the most CLs sent to them
+# will be listed.
+CL_SPLIT_TOP_REVIEWERS = 5
+
 
 def EnsureInGitRepository():
   """Throws an exception if the current directory is not a git repository."""
@@ -268,6 +273,7 @@ def SplitCl(description_file, comment_file, changelist, cmd_upload, dry_run,
     if answer.lower() != 'y':
       return 0
 
+    cls_per_reviewer = collections.defaultdict(int)
     for cl_index, (directory, files) in \
         enumerate(files_split_by_owners.items(), 1):
       # Use '/' as a path separator in the branch name and the CL description
@@ -284,6 +290,18 @@ def SplitCl(description_file, comment_file, changelist, cmd_upload, dry_run,
                  description, comment, reviewers, changelist, cmd_upload,
                  cq_dry_run, enable_auto_submit, topic, repository_root)
 
+      for reviewer in reviewers:
+        cls_per_reviewer[reviewer] += 1
+
+    # List the top reviewers that will be sent the most CLs as a result of the
+    # split.
+    reviewer_rankings = sorted(cls_per_reviewer.items(),
+                               key=lambda item: item[1],
+                               reverse=True)
+    print('The top reviewers are:')
+    for reviewer, count in reviewer_rankings[:CL_SPLIT_TOP_REVIEWERS]:
+      print(f'    {reviewer}: {count} CLs')
+
     # Go back to the original branch.
     git.run('checkout', refactor_branch)