Merge branch 'js/mingw-unc-path-w-backslashes'

In Git for Windows, "git clone \\server\share\path" etc. that uses
UNC paths from command line had bad interaction with its shell
emulation.

* js/mingw-unc-path-w-backslashes:
mingw: special-case arguments to `sh`
mingw (t5580): document bug when cloning from backslashed UNC paths