Notice index that has path and path/file and refuse to write such a tree.

Kay Sievers noticed that you can have both path and path/file in
the cache and write-tree happily creates a tree object from such
a state.  Since a merge can result in such situation and the
user should be able to see the situation by looking at the
cache, rather than forbidding add_cache_entry() to create such
conflicts, fix it by making write-tree refuse to write such an
nonsensical tree.  Here is a test case.

-- test case --

$ ls -a
./  ../
$ git-init-db
defaulting to local storage area
$ date >path
$ git-update-cache --add path
$ rm path
$ mkdir path
$ date >path/file
$ git-update-cache --add path/file
$ git-ls-files --stage
100644 1738f2536b1201218c41153941da065cc26174c9 0 path
100644 620c72f1c1de15f56ff9d63d6d7cdc69e828f1e3 0 path/file
$ git-ls-tree $(git-write-tree)                     ;# using old one
100644	blob	1738f2536b1201218c41153941da065cc26174c9	path
040000	tree	ec116937f223e3df95aeac9f076902ae1618ae98	path
$ ../git-write-tree                                 ;# using new one
You have both path and path/file
fatal: write-tree: not able to write tree
$ exit

Signed-off-by: Junio C Hamano <junkio@cox.net>

write-tree.c

@@ -84,7 +84,7 @@ static int write_tree(struct cache_entry **cachep, int maxentries, const char *b
 
 int main(int argc, char **argv)
 {
-	int i, unmerged;
+	int i, funny;
 	int entries = read_cache();
 	unsigned char sha1[20];
 
@@ -92,18 +92,45 @@ int main(int argc, char **argv)
 		die("write-tree: no cache contents to write");
 
 	/* Verify that the tree is merged */
-	unmerged = 0;
+	funny = 0;
 	for (i = 0; i < entries; i++) {
 		struct cache_entry *ce = active_cache[i];
 		if (ntohs(ce->ce_flags) & ~CE_NAMEMASK) {
-			if (++unmerged > 10) {
+			if (10 < ++funny) {
 				fprintf(stderr, "...\n");
 				break;
 			}
 			fprintf(stderr, "%s: unmerged (%s)\n", ce->name, sha1_to_hex(ce->sha1));
 		}
 	}
-	if (unmerged)
+	if (funny)
+		die("write-tree: not able to write tree");
+
+	/* Also verify that the cache does not have path and path/file
+	 * at the same time.  At this point we know the cache has only
+	 * stage 0 entries.
+	 */
+	funny = 0;
+	for (i = 0; i < entries - 1; i++) {
+		/* path/file always comes after path because of the way
+		 * the cache is sorted.  Also path can appear only once,
+		 * which means conflicting one would immediately follow.
+		 */
+		const char *this_name = active_cache[i]->name;
+		const char *next_name = active_cache[i+1]->name;
+		int this_len = strlen(this_name);
+		if (this_len < strlen(next_name) &&
+		    strncmp(this_name, next_name, this_len) == 0 &&
+		    next_name[this_len] == '/') {
+			if (10 < ++funny) {
+				fprintf(stderr, "...\n");
+				break;
+			}
+			fprintf(stderr, "You have both %s and %s\n",
+				this_name, next_name);
+		}
+	}
+	if (funny)
 		die("write-tree: not able to write tree");
 
 	/* Ok, write it out */