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 <[email protected]>

compat/mingw.c

@@ -1546,7 +1546,7 @@ static int is_msys2_sh(const char *cmd)
 		return ret;
 	}
 
-	if (ends_with(cmd, "\\sh.exe")) {
+	if (ends_with(cmd, "\\sh.exe") || ends_with(cmd, "/sh.exe")) {
 		static char *sh;
 
 		if (!sh)