Move --pretty options into Documentation/pretty-formats.txt

Move --pretty options into Documentation/pretty-formats.txt

Asciidoc-include it into the manuals for programs that use the
--pretty command-line option, for consistency among the docs.

This describes all the pretty-formats currently listed in the cmit_fmt
enum in commit.h, and also briefly describes the presence and format
of the 'Merge: ' line in some pretty formats.

There's a hedge that limiting your view of history can affect what
goes in the Merge: line, and that --abbrev/--no-abbrev do nothing to
the 'raw' format.

Signed-off-by: Chris Riddoch <chris@syntacticsugar.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>