A user may be in the habit of making many small commits,
and then squashing those down before uploading to Gerrit.
If this is the case, we should warn them before creating
20 changes when they meant to create only 1, as abandoning
all of those changes is still painful to do in the UI.
Add a per-user preference that caps the number of new
changes we will create in a single push command.
If the number of new changes would exceed that limit:
- abort early
- tell the user they are over-limit
- display the short-line description of each commit
- tell the user how they can increase their limit
By default the limit should be 1 for new users. Most
people are uploading one new change at a time.