s:fix_file_references: handle diff.mnemonicPrefix
s:fix_file_references: handle diff.mnemonicPrefix

An alternative would be to run `git` with no user config always, i.e.
setting `GIT_CONFIG_NOSYSTEM` and `HOME=/dev/null` (different for
Windows then) in its environment.  Maybe there's an easier way / single
option for this..

airblade/vim-gitgutter