mingw: use MSYS2 quoting even when spawning shell scripts

At the point where `mingw_spawn_fd()` is called, we already have a full
path to the script interpreter in that scenario, and we pass it in as
the executable to run, while the `argv` reflect what the script should
receive as command-line.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

diff --git a/compat/mingw.c b/compat/mingw.c
index 5dbaae9..32fc02e 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1439,7 +1439,8 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 	BOOL ret;
 	HANDLE cons;
 	const char *(*quote_arg)(const char *arg) =
-		is_msys2_sh(*argv) ? quote_arg_msys2 : quote_arg_msvc;
+		is_msys2_sh(cmd ? cmd : *argv) ?
+		quote_arg_msys2 : quote_arg_msvc;
 
 	do_unset_environment_variables();