commit: Fix specifier inheritance behavior

[Stephen J. Turnbull]

Michael Sperber writes:

 > "Stephen J. Turnbull" <stephen at xemacs.org> writes:
 > 
 > > Don't worry about changing Patcher's docs.  I'll just use them as a
 > > club to get Mike to fix the commit 'bot.
 > 
 > Just a naked club won't help.  What exactly would you like to see?

(1) I don't want to see merge turds.  The commit bot should be diffing
against the parent of the commit, not against the previous revision.
They're typically different in the case of merges.

(2) I do want to see commentary where appropriate.  Didier's patch
would have been much easier to understand if he wrote what he later
did in response to my query, and included it in the patch submission.