When cloning a repo that requires largefiles, the user had to either enable the
extension on the command line and then manually edit the local hgrc file after
the clone, or just enable it globally for the user. Since it is a feature of
last resort, and materially affects even repos without any largefiles when it is
enabled, we should make it easier to not have it enabled globally.
This simply adds the enabling statement to the local hgrc if the requires file
mandates its use (which only happens after the first largefile is committed).
That means that a user who works with a mix of largefile and normal repos can
always clone with '--config extensions.largefiles=', and the extension is
permanently enabled or not as appropriate.
The change in test-largefiles.t is simply because the order of loading rebase
and largefiles changed. The same change occurs if the order is flipped in the
hgrc file at the top of the test.
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