diff-hl-dired-{unknown,ignored}: inherit from dired-ignored instead
diff-hl-dired-update-ignores: Specifically exclude .hg from iteration

'hg status' throws a fit when passed that name.
Add user option `diff-hl-dired-extra-indicators'
Use "i" for ignored files
diff-hl-dired: Always use dir-status-files

...halving the number of process calls.
Introduce diff-hl-dired-mode-unless-remote

Closes #31