Follow-up to #46001 

That initial fix partly addressed the issue for diffs managed by the
`GitStore`, but not for other diffs (e.g. those managed by the
`ActionLog` or `CommitView`). The underlying issue is that we switched
to using a `Buffer` to represent the diff base text, and when updating
the diff we were calling `set_text` on this buffer and not waiting for
reparsing to finish. When the base text was represented by a series of
independent `BufferSnapshot`s, this wasn't an issue because we would
parse the base text in the background as part of computing the diff
update. This PR fixes the issue by waiting on reparsing to finish after
each call to `set_text`.

Release Notes:

- N/A