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: NJunio C Hamano <junkio@cox.net>
Acked-by: NStefan Richter <stefanr@s5r6.in-berlin.de>

Documentation/asciidoc.conf

@@ -11,6 +11,7 @@
 caret=^
 startsb=[
 endsb=]
+tilde=~
 
 ifdef::backend-docbook[]
 [gitlink-inlinemacro]

Documentation/git-rev-parse.txt

@@ -138,7 +138,7 @@ syntax.
   'rev{caret}0' means the commit itself and is used when 'rev' is the
   object name of a tag object that refers to a commit object.
 
-* A suffix '~<n>' to a revision parameter means the commit
+* A suffix '{tilde}<n>' to a revision parameter means the commit
   object that is the <n>th generation grand-parent of the named
   commit object, following only the first parent.  I.e. rev~3 is
   equivalent to rev{caret}{caret}{caret} which is equivalent to\