Add textobj-diff to the configuration.
Ensure file is not named 'index' file before DiffGitCached.
Merge branch 'js/add-textobj-diff' into add-textobj-diff
Remove the many global mappings for textobj-diff.

Replace them with a few buffer-local mappings that apply only
for 'diff' files and eliminate 'dH' as an alias for 'df' (diff
file), to conserve mappable keys.
Sync with upstream.