magit-mark-commit: use magit-mode-get-buffers

instead of `magit-map-magit-buffers' which was just removed.
magit-start-process: respect inhibit-magit-refresh

Dynamically binding `inhibit-magit-refresh' to t previously had no
effect for asynchronous processes because the refresh is triggered
by `magit-process-sentinel' which is run outside that binding.

Now `magit-start-process' saves the value of `inhibit-magit-refresh'
in a property of the process and `magit-process-sentinel' forgoes
calling `magit-refresh' if that is non-nil.
magit-blame: bind inhibit-magit-refresh to t

Without this the blame information is created and then immediately
destroyed again in `magit-revert-buffers' called by `magit-refresh'.
magit-blame-process-filter: avoid empty lines

When splitting partial output from `git blame --incremental' drop empty
lines, i.e. very partial output which we cannot process in a meaningful
way.  There are not actually empty lines in the output, but we might
receive partial output which only consists of a newline.  Dropping the
newline does not cause any problems latter when we combine partial
output, because we store partial output as a list of lines.