Sanity check: make sure that trailing slashes are present on source dir when target is a directory

[fredemmott]

Eg Map { 'foo' => '' } is invalid, because then foo/bar becomes /bar instead of bar.

[sdwilsh]

4fd8a55 updated all Facebook projects to check for this. I think there are a few tests that cover this now as well, but nothing in the stock ShipIt.