Documentation: remove backslashes in manpage synopses

Documentation: remove backslashes in manpage synopses

For some reason, various manual pages have an asterisk escaped
with \ in the synopsis.  Since there is no other asterisk to pair it
with, Asciidoc does not consider this asterisk escapable, so it passes
the backslash through.

Each page either uses [verse] or has only one asterisk, so it
is safe to drop the backslashes (checked with asciidoc 8.5.2).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>