Log improvements

Log command is now able to display history of repos with branches, and has some additional options for filtering.

An operation for managing parsing of timestamp strings has been included, which can be used by other operations as well.

The user manual now describes this new functions. (it still misses some examples, but work on them can be done once merged)

Things to improve from here:
- The --topo-order performance might be improved by using the reflog to see when a branch is created, instead of checking for commits with multiple children
- The --summary option can be improved by using diff printers that are being developed in the patch-conflicts branch