escape tilde in Documentation/git-rev-parse.txt

escape tilde in Documentation/git-rev-parse.txt

Fixes a failure to build the git-rev-parse manpage, seen with
asciidoc 8.0.0

We would love to use nicer quoting $$~$$ but alas asciidoc 7
does not know about it.  So use asciidoc.conf and define {tilde}
to be ~.

Signed-off-by: Junio C Hamano <junkio@cox.net>
Acked-by: Stefan Richter <stefanr@s5r6.in-berlin.de>