Clarify commit error message for unmerged files

Currently, trying to use git-commit with unmerged files in the index
will show the message "Error building trees", which can be a bit
obscure to the end user. This patch makes the error message clearer, and
consistent with what git-write-tree reports in a similar situation.

Signed-off-by: Rafael Garcia-Suarez <rgarciasuarez@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

builtin-commit.c

@@ -639,7 +639,7 @@ static int prepare_to_commit(const char *index_file, const char *prefix)
 		active_cache_tree = cache_tree();
 	if (cache_tree_update(active_cache_tree,
 			      active_cache, active_nr, 0, 0) < 0) {
-		error("Error building trees");
+		error("Error building trees; the index is unmerged?");
 		return 0;
 	}