`magit-process-keep-history`: optionally don't clear the process buffer

Most calls to `magit-run*` don't set its NOERASE arg to a non-nil value,
so the process buffer is cleared pretty much every time we run a command
that makes use of `magit-run*`.

However, sometimes it's useful to be able to refer back to the log
of recent Git commands (and their output), e.g. when cleaning up stale
remote-tracking branches after having fetched with the "--prune" flag.

Now we can prevent the process buffer from ever being cleared by setting
`magit-process-keep-history` to a non-nil value. To improve readability,
individual command invocations will be delimited with an empty line.

Signed-off-by: Pieter Praet <pieter@praet.org>