Make error messages from cherry-pick/revert more sensible

The original "rewrite in C" did somewhat a sloppy job while
stealing code from git-write-tree.

The caller pretends as if the write_tree() function would return
an error code and being able to issue a sensible error message
itself, but write_tree() function just calls die() and never
returns an error.  Worse yet, the function claims that it was
running git-write-tree (which is no longer true after
cherry-pick stole it).

Tested-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.c

@@ -529,3 +529,58 @@ struct cache_tree *cache_tree_find(struct cache_tree *it, const char *path)
 	}
 	return it;
 }
+
+int write_cache_as_tree(unsigned char *sha1, int missing_ok, const char *prefix)
+{
+	int entries, was_valid, newfd;
+
+	/*
+	 * We can't free this memory, it becomes part of a linked list
+	 * parsed atexit()
+	 */
+	struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
+
+	newfd = hold_locked_index(lock_file, 1);
+
+	entries = read_cache();
+	if (entries < 0)
+		return WRITE_TREE_UNREADABLE_INDEX;
+
+	if (!active_cache_tree)
+		active_cache_tree = cache_tree();
+
+	was_valid = cache_tree_fully_valid(active_cache_tree);
+
+	if (!was_valid) {
+		if (cache_tree_update(active_cache_tree,
+				      active_cache, active_nr,
+				      missing_ok, 0) < 0)
+			return WRITE_TREE_UNMERGED_INDEX;
+		if (0 <= newfd) {
+			if (!write_cache(newfd, active_cache, active_nr) &&
+			    !commit_lock_file(lock_file))
+				newfd = -1;
+		}
+		/* Not being able to write is fine -- we are only interested
+		 * in updating the cache-tree part, and if the next caller
+		 * ends up using the old index with unupdated cache-tree part
+		 * it misses the work we did here, but that is just a
+		 * performance penalty and not a big deal.
+		 */
+	}
+
+	if (prefix) {
+		struct cache_tree *subtree =
+			cache_tree_find(active_cache_tree, prefix);
+		if (!subtree)
+			return WRITE_TREE_PREFIX_ERROR;
+		hashcpy(sha1, subtree->sha1);
+	}
+	else
+		hashcpy(sha1, active_cache_tree->sha1);
+
+	if (0 <= newfd)
+		rollback_lock_file(lock_file);
+
+	return 0;
+}