Remove sed date transformation in pretty_git_log

I think I'd actually prefer:

- (2 years ago) instead of (2 years)
- (2 years, 5 months) instead of (2 years)
Simplify pretty_git_log and show_git_head

I extracted them to bash scripts from a common file but never got around
to cleaning up the duplication. Until now.
Some git aliases for analyzing repo authors
Initial git-merges: show formatted log of merges