use "sentinel" function attribute for variadic lists

This attribute can help gcc notice when callers forget to
add a NULL sentinel to the end of the function. This is our
first use of the sentinel attribute, but we shouldn't need
to #ifdef for other compilers, as __attribute__ is already a
no-op on non-gcc-compatible compilers.

Suggested-by: Bert Wesarg <bert.wesarg@googlemail.com>
More-Spots-Found-By: Matt Kraai <kraai@ftbfs.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

exec_cmd.h

@@ -7,6 +7,7 @@ extern const char *git_exec_path(void);
 extern void setup_path(void);
 extern const char **prepare_git_cmd(const char **argv);
 extern int execv_git_cmd(const char **argv); /* NULL terminated */
+__attribute__((sentinel))
 extern int execl_git_cmd(const char *cmd, ...);
 extern const char *system_path(const char *path);