$ cat samefile.c
/* samefile -- decides whether two files given as arguments are the same.
follows symlinks. both files must be valid and you must have permission 
to read them, and even then it may fail with an error or hang.
exit code 0 is same, 1 if different, higher on an error.
*/
#include <unistd.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/wait.h>
int
main(int argc, char *argv[]) {
	int d0, d1, sch;
	struct flock lk;
	pid_t ppa, pch;
	if (3 != argc) return 10;
	if (-1 == (d0 = open(argv[1], O_WRONLY))) return 4;
	if (-1 == (d1 = open(argv[2], O_RDONLY))) return 3;
	lk.l_type = F_WRLCK;
	lk.l_whence = SEEK_SET;
	lk.l_start = 0;
	lk.l_len = 1;
	if (-1 == fcntl(d0, F_SETLK, &lk)) return 5;
	ppa = getpid();
	if (-1 == (pch = fork())) return 9;
	if (!pch) {
		lk.l_type = F_RDLCK;
		lk.l_whence = SEEK_SET;
		lk.l_start = 0;
		lk.l_len = 1;
		if (-1 == fcntl(d1, F_GETLK, &lk)) return 4;
		return !(F_WRLCK == lk.l_type && ppa == lk.l_pid);
	} else {
		if (pch != wait(&sch)) return 8;
		if (!WIFEXITED(sch)) return 7;
		return WEXITSTATUS(sch);
	}
}
$ gcc -Wall -O -o samefile samefile.c 
$ ./samefile /usr/local/libexec/git-core/git-{diff,pull}; echo $?
1
$ ./samefile /usr/local/libexec/git-core/git-{diff,commit}; echo $?
0
$ 

##</code><code>##

$ test /usr/local/libexec/git-core/git-diff -ef /usr/local/libexec/git-core/git-pull; echo $?
1
$ test /usr/local/libexec/git-core/git-diff -ef /usr/local/libexec/git-core/git-commit; echo $?
0