manpages: italicize nongit command names (if they are in teletype font)

manpages: italicize nongit command names (if they are in teletype font)

Some manual pages use teletype font to set command names. We
change them to use italics, instead.  This creates a visual
distinction between names of commands and command lines that
can be typed at the command line. It is also more consistent
with other man pages outside Git.

In this patch, the commands named are non-git commands like bash.

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