Pylint uses parallelism to improve performance, but high startup costs
means that this only makes sense if there are lots of files to be
processed. So, a while ago a change was made such that if there are
fewer than ten files to be analyzed then no parallelism is used.

Our Pylint wrapper also has a hack where it does one type of check in
serial mode, because that is the only time it is reliable. This
requires running Pylint twice, which is expensive.

If there are a small enough number of files to analyze then we will be
doing serial analysis anyway, so there is no need to do two separate
runs. In this test case:

  git cl presubmit -v --files tools\code_coverage\create_js_source_maps\test\create_js_source_maps_test.py

the cost of Pylint is dropped roughly in half, from six seconds to
three seconds, by eliminating one of the three-second runs.

Bug: 1309977
Change-Id: I2e5e96a86d1d76b127f481af7478d807c042b609
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3812436Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>