fix git commit --amend -m "new message"

fix git commit --amend -m "new message"

The prepare_log_message() function serves two purposes:

 - Prepares the commit log message template, to be given to the end
   user;

 - Return true if there is something committable;

7168624c3530d8c7ee32f930f8fb2ba302b9801f (Do not generate full commit
log message if it is not going to be used) cheated to omit the former
when we know the log message template is not going to be used.  However,
its replacement logic to see if there is something committable was
botched.  When amending, it should compare the index with the parent of
the HEAD, not the current HEAD.  Otherwise you cannot run --amend to
fix only the message without changing the tree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>