[codeguard/codeguard#1050] Change truncating policy

1. Previously, given a repo with 4 commits and a cutoff date in the
middle of the 4 commits, git-tail would say 'Truncating 2 commits'
but after execution, 3 commits would remain.

Now, use new_log_entries.last as the old_base, so that truncating 2
commits in a repo with 4 commits means that 2 will be removed and 2
will remain.

2. Previously, if all of the commits occurred before the cutoff date
git-tail would 'combine' (not sure what the terminology is) all of them
leaving only one commit after execution.

Now, the user has to be more explicit. In the above case git-tail will
abort and output, "No commits after cutoff date. Skipping." To combine
all commits the user would have to give a cutoff date between the most
recent and 2nd most recent commits.