Documentation: Split description of pretty formats of commit log

Documentation: Split description of pretty formats of commit log

Split description of pretty formats into list of pretty options
(--pretty and --encoding) in new file Documentation/pretty-options.txt
and description of formats itself as a separate "PRETTY FORMATS"
section in pretty-formats.txt

While at it correct formatting a bit, to be better laid out in the
resulting manpages: git-rev-list(1), git-show(1), git-log(1) and
git-diff-tree(1).  Those manpages now include pretty options in the
same place as it was before, and description of formats just after
all options.

Inspired by the split into two filesdocumentation for merge strategies:
Documentation/merge-options.txt and Documentation/merge-strategies.txt

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>