Improve git aliases (fewer, but better)

Cut out some unused aliases, improve output from log and for-each-ref
aliases, and add some new aliases to list heads, tags, remotes etc.

Note: some of these aliases require git version 1.9+ as they use the
colour formatting options with for-each-ref, which were introduced in
version 1.9.0. They could be rewritten to use escape sequences though.