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