Ignore rebase merges to avoid double (or more) counting of lines

A giant rebase squash I did today sent a huge number of redundant commit lines to the collector.  This should prevent that by detecting the .git/rebase-merge directory, which is present during rebase merges. 

http://stackoverflow.com/questions/11367722/git-post-commit-skip-amend-and-rebase

I'm not sure of the wider design goals of Zeitgit, but leaving those lines out seems like a better representation of coding activity.  I suppose an alternate would be to send a commit message with 0 lines.