git-[short]log.txt: unify quoted standalone --

git-[short]log.txt: unify quoted standalone --

In git-log.txt, we have an instance of \--, which is known to sometimes
render badly. This one is even worse than normal though, since ``\-- ''
(with or without that trailing space) appears to be entirely broken,
both in HTML and manpages, both with AsciiDoc (version 8.6.9) and
Asciidoctor (version 1.5.4).

Further down in git-log.txt we have a ``--'', which renders good. In
git-shortlog.txt, we use "\-- " (including the quotes and the space),
which happens to look fairly good. I failed to find any other similar
instances. So all in all, we quote a double-dash in three different
places and do it differently each time, with various degrees of success.

Switch all of these to `--`. This sets the double-dash in monospace and
matches what we usually do with example command line usages and options.
Note that we drop the trailing space as well, since `-- ` does not
render well. These should still be clear enough since just a few lines
above each instance, the space is clearly visible in a longer context.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>