Merge branch 'tb/clone-case-smashing-warning-test'

The code recently added to "git clone" to see if the platform's
filesystem is adequate to check out and use the project code
correctly (e.g. a case smashing filesystem cannot be used for a
project with two files whose paths are different only in case) was
meant to help Windows users, but the test for it was not enabled
for that platform, which has been corrected.

* tb/clone-case-smashing-warning-test:
  t5601-99: Enable colliding file detection for MINGW
