Add a running log of git command output.

As well as the magit-process buffer (showing the output of the most recent git command), it would be mightily useful if that output was also appended to a magit-process-log buffer, so that we could see things like the commit hash for the stash that we dropped a couple of commands ago, and then realised that we wanted back (for example :)

https://github.com/phil-s/magit/commit/6d0b67d57da8949bd424cf9450a2802da0c828fb

Branch phil-s/process-log (branched from maint) implements this.