write-tree: --prefix=<path>

The "bind" commit can express an aggregation of multiple
projects into a single commit.

In such an organization, there would be one project, root of
whose tree object is at the same level of the root of the
aggregated projects, and other projects have their toplevel in
separate subdirectories.  Let's call that root level project the
"primary project", and call other ones just "subprojects".

You would first read-tree the primary project, and then graft
the subprojects under their appropriate location using read-tree
--prefix=<subdir>/ repeatedly.

To write out a tree object from such an index for a subproject,
write-tree --prefix=<subdir>/ is used.

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

cache-tree.c

@@ -525,3 +525,29 @@ struct cache_tree *cache_tree_read(const char *buffer, unsigned long size)
 		return NULL; /* not the whole tree */
 	return read_one(&buffer, &size);
 }
+
+struct cache_tree *cache_tree_find(struct cache_tree *it, const char *path)
+{
+	while (*path) {
+		const char *slash;
+		struct cache_tree_sub *sub;
+
+		slash = strchr(path, '/');
+		if (!slash)
+			slash = path + strlen(path);
+		/* between path and slash is the name of the
+		 * subtree to look for.
+		 */
+		sub = find_subtree(it, path, slash - path, 0);
+		if (!sub)
+			return NULL;
+		it = sub->cache_tree;
+		if (slash)
+			while (*slash && *slash == '/')
+				slash++;
+		if (!slash || !*slash)
+			return it; /* prefix ended with slashes */
+		path = slash;
+	}
+	return it;
+}