Log formatting fix
Change the default grep-command to use "git grep"
Store resetBranches and deletedBranches fragments in a hash table

Preparing to avoid creating branches and deleting them in the same revision
When a branch is added and then deleted in the same revision, make it a no-op

This happens often in the Boost repository where someone branches the
whole repository and then goes about deleting subdirectories in the
branch.

Addresses, but doesn't completely fix, issue #6