Replace 'Mercurial' in What's New

[terryjreedy]

@benjaminp, @pablogsal
The boilerplate at the top of doc/whatsnew/3.12.rst, just readded by Benjamin, and the same for 3.11 and 3.9 have, at about line 44, an obsolete reference to Mercurial. (The only one in the file at least in 3.12.)

   This saves the maintainer the effort of going through the Mercurial log
   when researching a change.

3.10 has 'git log'. 3.8 and 3.7 have 'Git log'.

Should it consistently be 'git' or 'Git', or does the 'whatsnew' maintainer even look at the checkin logs versus the news 'changelog', referred to a couple of lines, or is there even a 'whatsnew maintainer' in the sense used in the text?

PS: 3.12, only, has an extraneous blank line at the top.