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

[Eli Zaretskii]

> From: "Jan D." <jan.h.d@swipnet.se>
> Date: Sat, 18 Apr 2015 09:46:37 +0200
> Cc: 20325@debbugs.gnu.org, Paul Eggert <eggert@cs.ucla.edu>
> 
> The hard part is to force all contributors to change their habits.

Which in this particular case I had to do already 3 times since last
November, with the 4th time looming.