mingw(is_msys2_sh): handle forward slashes in the `sh.exe` path, too

Whether the full path to the MSYS2 Bash is specified using backslashes or forward slashes, in either case the command-line arguments need to be quoted in the MSYS2-specific manner instead of using regular Win32 command-line quoting rules. In preparation for `prepare_shell_cmd()` to use the full path to `sh.exe` (with forward slashes for consistency), let's teach the `is_msys2_sh()` function about this; Otherwise 5580.4 'clone with backslashed path' would fail once `prepare_shell_cmd()` uses the full path instead of merely `sh`. This patch relies on the just-introduced fix where `fspathcmp()` handles backslashes and forward slashes as equivalent on Windows. Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>