Add "--color" into magit-log mode

I wanted magit-log to be a little clearer when there were branch lines that cross through each other in magit-log.  Since "git log" already supports colored branch lines, I looked into how I could take advantage of it.

This change uses the build in ansi-color.el's ansi-color-apply to translate "git log --color"'s color control sequences into text properties so they can be displayed with color in the magit-log buffer.

While it works, this change doesn't feel "complete", for example I left magit-log-graph orphaned and it would be nice if it was still available for use.  Also, I'm not confident that the ansi-color-apply usage here is idiomatic or covers all scenarios (I tested with a couple.. light and dark background).  I think this being my first attempt at tinkering with elisp would have something to do with it.  Any feedback about this change (or the process.. felt a little weird to submit this as a pull request) would be super welcome.   Thanks.