tools/pipbot: Support short time deltas

If we have a time delta that is less than an hour ago, we weren't
returning anything for the time string.
Example:

    $ pipbot burndown release
    Comparing 'master' against 'stable'...

      There are 5 new commits
      Last commit was  ago

This adds support for short time spans (less than one hour).
Example:

    $ pipbot burndown release
    Comparing 'master' against 'stable'...

      There are 5 new commits
      Last commit was 27 minutes ago
tools/pipbot: Always print the last commit time

Even though we may not have made a commit, it's still useful to know
when the last commit was.
tools/pipbot: Don't print '0 minutes'

For time deltas of less than a minute, this means that only the seconds
are printed, instead '0 minutes, X seconds'.
Example:

   $ pipbot burndown
   Comparing 'HEAD' against 'master'...

      There are no new commits
      Last commit was 0 minutes, 7 seconds ago
tools/pipbot: Correctly pluralise single units

Example: "1 minute" vs "1 minutes"