Don't use a transaction in cursor/selection movement commands

Previously, a transaction was used to set an undo grouping interval
for every editor command except `undo` and `redo`. The problem is
that currently, moving a marker inside of a transaction causes the
buffer's redo stack to be cleared. For now, just don't use
transactions for commands that don't modify the buffer's contents."
Merge pull request #4455 from atom/mb-fix-redo

Don't use a transaction in cursor/selection movement commands