A faster way to get modification dates

[timsong-cpp]

This runs in <1.5s on my box.

Only delta compared to the current method is four files that no longer exist, but that should be harmless.

1336d1335
< 1336 1289151416
1341,1343d1339
< 1341 1289151416
< 1342 1289151416
< 1343 1289151416

[Dani-Hub]

Thanks Tim!

[jwakely]

This is great, I didn't know about whatchanged (although I suppose the same would be achievable with log --pretty with a bit more work).