main: use custom pretty format to speed up loading

In Git's repository, the difference in output between:

 $ git log --parents --pretty=raw
 $ git log --date=raw --parents --pretty="format:commit %H %P%x00%an <%ae> %ad%x00%s"

is ~26M vs ~7M. Using this simplified format speeds up loading from 28s
to 20s with graph enabled and from ~4 to less than 3 seconds without the
graph.

The pretty raw format is still supported to allow piping reflog data to
the main view.