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.h

@@ -30,4 +30,9 @@ int cache_tree_update(struct cache_tree *, struct cache_entry **, int, int, int)
 
 struct cache_tree *cache_tree_find(struct cache_tree *, const char *);
 
+#define WRITE_TREE_UNREADABLE_INDEX (-1)
+#define WRITE_TREE_UNMERGED_INDEX (-2)
+#define WRITE_TREE_PREFIX_ERROR (-3)
+
+int write_cache_as_tree(unsigned char *sha1, int missing_ok, const char *prefix);
 #endif