Fix users of prefix_path() to free() only when necessary

Unfortunately, prefix_path() sometimes returns a newly xmalloc()ed buffer,
and in other cases it returns a substring!

For example, when calling

	git update-index ./hello.txt

prefix_path() returns "hello.txt", but does not allocate a new buffer. The
original code only checked if the result of prefix_path() was different from
what was passed in, and thusly trigger a segmentation fault.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

checkout-index.c

@@ -277,7 +277,7 @@ int main(int argc, char **argv)
 			die("git-checkout-index: don't mix '--stdin' and explicit filenames");
 		p = prefix_path(prefix, prefix_length, arg);
 		checkout_file(p);
-		if (p != arg)
+		if (p < arg || p > arg + strlen(arg))
 			free((char*)p);
 	}
 
@@ -299,7 +299,7 @@ int main(int argc, char **argv)
 				path_name = buf.buf;
 			p = prefix_path(prefix, prefix_length, path_name);
 			checkout_file(p);
-			if (p != path_name)
+			if (p < path_name || p > path_name + strlen(path_name))
 				free((char *)p);
 			if (path_name != buf.buf)
 				free(path_name);

update-index.c

@@ -382,7 +382,7 @@ static void update_one(const char *path, const char *prefix, int prefix_length)
 		die("Unable to process file %s", path);
 	report("add '%s'", path);
  free_return:
-	if (p != path)
+	if (p < path || p > path + strlen(path))
 		free((char*)p);
 }
 
@@ -594,7 +594,7 @@ static int do_unresolve(int ac, const char **av,
 		const char *arg = av[i];
 		const char *p = prefix_path(prefix, prefix_length, arg);
 		err |= unresolve_one(p);
-		if (p != arg)
+		if (p < arg || p > arg + strlen(arg))
 			free((char*)p);
 	}
 	return err;
@@ -750,7 +750,7 @@ int main(int argc, const char **argv)
 			update_one(p, NULL, 0);
 			if (set_executable_bit)
 				chmod_path(set_executable_bit, p);
-			if (p != path_name)
+			if (p < path_name || p > path_name + strlen(path_name))
 				free((char*) p);
 			if (path_name != buf.buf)
 				free(path_name);