[PATCH 0 of 2 v3] scmutil/add: warnings for non-portable case-folding collisions

[Kevin Gessner]

Compared to v2 of this patch:

- for efficiency, portability checks are not done if the user has turned off
the warnings with ui.portablefilenames.

- test case for the case-folding warnings. It's skipped on case-insensitive
filesystems; I used a read/write case-sensitive disk image to test it on my
case-insensitive Mac.