Build: Reduce the slim build header comment & jQuery.fn.jquery

Build: Reduce the slim build header comment & jQuery.fn.jquery

So far, the slim build was expanded to its full exclusion list, generating the
following `jQuery.fn.jquery`:
```
v4.0.0-pre -ajax,-ajax/jsonp,-ajax/load,-ajax/script,-ajax/var/location,-ajax/var/nonce,-ajax/var/rquery,-ajax/xhr,-manipulation/_evalUrl,-deprecated/ajax-event-alias,-callbacks,-deferred,-deferred/exceptionHook,-effects,-effects/Tween,-effects/animatedSelector,-queue,-queue/delay,-core/ready
```

This commit changes it to just `v4.0.0-pre slim`. Only the pure slim build is
treated this way, any modification to it goes through the old expansion; e.g.
for `custom:slim,-deprecated` we get the following `jQuery.fn.jquery`:
```
v4.0.0-pre -deprecated,-deprecated/ajax-event-alias,-deprecated/event,-ajax,-ajax/jsonp,-ajax/load,-ajax/script,-ajax/var/location,-ajax/var/nonce,-ajax/var/rquery,-ajax/xhr,-manipulation/_evalUrl,-callbacks,-deferred,-deferred/exceptionHook,-effects,-effects/Tween,-effects/animatedSelector,-queue,-queue/delay,-core/ready
```

Since the version string is also put in the jQuery header comment, it also got
smaller.

Also, the logic to skip including the commit hash in the header comment - when
provided through the COMMIT environment variable which we do in Jenkins - in
minified builds headers has been applied to builds with exclusions as well.

Closes gh-4649