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>

builtin.h

@@ -8,7 +8,6 @@ extern const char git_usage_string[];
 
 extern void list_common_cmds_help(void);
 extern void help_unknown_cmd(const char *cmd);
-extern int write_tree(unsigned char *sha1, int missing_ok, const char *prefix);
 extern void prune_packed_objects(int);
 
 extern int cmd_add(int argc, const char **argv, const char *prefix);