Scanners can return errors (e.g. ErrTooLong), and if we don't handle it, the
cmd.Wait() call below will block forever because nobody drains the command's
output.

This happens for CommitLoader.GetCommits when there's a commit whose subject
line is longer than approx. 65500 characters; in that case, lazygit would lock
up completely. With this fix it remains usable, but the commit list is truncated
before the bad commit, which is not good enough. We'll improve that in the
remaining commits of this branch.

We are going to truncate overly long lines returned from git log, and the most
likely field that is going to make the line too long is the subject; so we must
put it last, otherwise we'd end up with not enough fields to split when it's too
long.

It might not be obvious from the diff what's happening to the mock command
output in the test: it didn't have the divergence field (">") at all, which was
kind of a bug. It didn't matter for these tests though, because we are not
testing the divergence here, and our production code happens to be resilient
against it missing. But now we must add the ">" field before the subject.