git-map: Refactor and add simple tests

Change-Id: I8fd0034f6a6d7623792620f92208b25961fa174e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1990142
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

PRESUBMIT.py

@@ -73,6 +73,7 @@ def CommonChecks(input_api, output_api, tests_to_black_list, run_on_python3):
         r'.*git_cl_test\.py$',
         r'.*git_common_test\.py$',
         r'.*git_hyper_blame_test\.py$',
+        r'.*git_map_test\.py$',
         r'.*git_number_test\.py$',
         r'.*git_rebase_update_test\.py$',
         r'.*ninjalog_uploader_test\.py$',