MinGW: truncate exit()'s argument to lowest 8 bits

For some reason, MinGW's bash cannot reliably detect failure of the child
process if a negative value is passed to exit(). This fixes it by
truncating the exit code in all calls of exit().

This issue was worked around in run_builtin() of git.c (2488df84 builtin
run_command: do not exit with -1, 2007-11-15). This workaround is no longer
necessary and is reverted.
Suggested-by: NJunio C Hamano <gitster@pobox.com>
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/mingw.h

@@ -92,6 +92,8 @@ static inline int fcntl(int fd, int cmd, long arg)
 	errno = EINVAL;
 	return -1;
 }
+/* bash cannot reliably detect negative return codes as failure */
+#define exit(code) exit((code) & 0xff)
 
 /*
  * simple adaptors

git.c

@@ -245,7 +245,7 @@ static int run_builtin(struct cmd_struct *p, int argc, const char **argv)
 
 	status = p->fn(argc, argv, prefix);
 	if (status)
-		return status & 0xff;
+		return status;
 
 	/* Somebody closed stdout? */
 	if (fstat(fileno(stdout), &st))