bug#20325: convert "fixes:debbugs:123" in log to (Bug#123) in ChangeLog

[Eli Zaretskii]

> Date: Fri, 17 Apr 2015 18:02:51 -0700
> From: Paul Eggert <eggert@cs.ucla.edu>
> Cc: 20325@debbugs.gnu.org
> 
> "Fixes: debbugs:NNNN" is designed for Bzr I think,

Bzr didn't have this in the log messages, it had this as meta-data.
Conversion to Git moved this into the commit log, since AFAIK Git
doesn't have such an attribute in its meta-data.