Merge branch 'da/mergetools-special-case-xxdiff-exit-128'

The xxdiff difftool backend can exit with status 128, which the
difftool-helper that launches the backend takes as a significant
failure, when it is not significant at all.  Work it around.

* da/mergetools-special-case-xxdiff-exit-128:
  mergetools/xxdiff: prevent segfaults from stopping difftool