git-g*: also recognize Cygwin-flavored Perl on Windows

The MSYS2-based Perl shipped with recent Git for Windows reports $^O eq
"cygwin" rather than "msys", so the path normalization in the BEGIN
block was skipped. abs_path($0) on a Windows-style path then got
prepended with the cwd, breaking the @INC entry used to locate
git_gpush.pm. Accept "cygwin" as well.

The change to Perl reporting "cygwin" is in the MSYS2 SDK that's used
for building Git for Windows. See
https://github.com/msys2/MSYS2-packages/issues/3401

Change-Id: Iec21414d720e073c509a10ffb67ffbcd5f589717
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>