Trigger OptionSet autocommand on entering diff mode

[Christian Brabandt]

Bram,
currently, when entering diff mode, the OptionSet autocommand is not
called.

The attached patch changes that, so that a user can set specific options
only for diffmode.

Also the oldstyle test was converted to a new style test (and a small
bug in the oldstyle test was fixed).

Please see also the commit description in the attached patch for more
information.

And while I was writing the test, I noticed that test_override() was
missing from usr_41.txt, so I added it there as well.

Best,
Christian

[Bram Moolenaar]

Thanks.

Did you check that something nasty in the autocommand, such as closing
the buffer, doesn't cause trouble?

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[Christian Brabandt]

On Fr, 07 Jul 2017, Bram Moolenaar wrote:

> Did you check that something nasty in the autocommand, such as closing
> the buffer, doesn't cause trouble?

This version does, by setting curbuf_locked. That makes at least my
initial test:
au OptionSet diff :close
not fail (and still enters diff mode). (previously it would segfault.)

Best,
Christian
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