Fix modified files list in webhooks when there is a space (#16288)

Fix modified files list in webhooks when there is a space (#16288)

* Fix modified files list in webhooks when there is a space

There is an unfortunate bug with GetCommitFileStatus where files with
spaces are misparsed and split at the space.

There is a second bug because modern gits detect renames meaning that
this function no longer works correctly.

There is a third bug in that merge commits don't have their modified
files detected correctly.

Fix #15865

Signed-off-by: Andrew Thornton <art27@cantab.net>