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>