As reported several times in issue #1006, the dot-command
no longer repeats insertions since roughly Atom release 1.7.0.
This can be traced back to an update in the text-buffer package,
which appears to have changed the behavior of
getChangesSinceCheckpoint in lib/vim-state.coffee and the
TransactionBuilder class in input.coffee which uses its return.

This commit replaces the local getChangesSinceCheckpoint function
by the counterpart of the same name from the text-buffer package,
rendering the local implementation obsolete. The TransactionBuilder
class is rewritten to deal with this alternate input.

The getChangesSinceCheckpoint function from the text-buffer package
already bundles the changes since the last checkpoint, so there is
no need for a seperate TransactionBundler implementation here.

In fact, the previous implementation overbundled changes by bundling
the already bundled edits, causing bugs when using multiple cursors.
This commit fixes that issue by working directly with the return
from buffer.getChangesSinceCheckpoint.