The routine to determine which files to check for candidacies, when
used to analyze files being changed with --HEAD, should only test
those files if they are being kept by the change. If they're being
deleted, then there is no reason to test them. Further, testing
files which are slated for removal makes it basically impossible to
revert a previously merged candidacy which has since been
invalidated (for example, by resigning OSF Individual Membership).
Mock os.path.exists in test_find_modified_candidate_files_all_good
to just return True, since we aren't actually creating the file in
that test.
Change-Id: I4d5d9b1d55379adfd1c7d7765324306abf40cdb7
4570981c1b