MSYS and MSYS2 environments (such as Git Bash) have a UNIX like
filesystem which uses '/' as the path separator rather than '\', but
Rust doesn't know about this by default.
On Windows, check the MSYSTEM environment variable and set the default
value of the --path-separator option to '/' for convenience.
There is no similar detection of Cygwin because there seems to be no way
for Rust (and any native Win32) programs to detect that they're being
called from a Cygwin environment. Cygwin users can use a shell
alias/function/script to wrap fd.
Fixes: https://github.com/sharkdp/fd/issues/537
