doc: Add back quotes around filenames
Filenames starting with a dot (.hg and .hgignore) confuse man when
creating the ps documentation with "man -t hg >hg.ps" if they are not
enclosed in back quotes.
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ echo '[extensions]' >> $HGRCPATH
$ echo "gpg =" >> $HGRCPATH
$ echo "hgext.gpg =" >> $HGRCPATH
$ echo "badext = $abspath" >> $HGRCPATH
$ echo "badext2 =" >> $HGRCPATH
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
hg help [TOPIC]
show help for a given topic or a help overview