Remove OS-specific commands, change default for git option

I've been using this plugin for quite some time, and it works very well.

Some changes were necessary to make it work on different systems. I didn't send a pull request right away because I intended to test it for some time, but I end up forgetting about it :)
My setting is very similar to the default, mainly removed the %d in order to have a log file per month. I don't remember the exact reason I inserted the `expand()` call -- maybe the `~` didn't expand correctly on some system.