This is conceptually very simple, just making a 1 that was hard coded be
exposed as a config option. The hard part was plumbing all that, and
dealing with complexities like reading it from git attributes at the
same time that numcopies is read.
Behavior change: When numcopies is set to 0, git-annex used to drop
content without requiring any copies. Now to get that (highly unsafe)
behavior, mincopies also needs to be set to 0. It seemed better to
remove that edge case, than complicate mincopies by ignoring it when
numcopies is 0.
This commit was sponsored by Denis Dzyubenko on Patreon.
Looked into this, and dropKey from web actually removes the url,
so git-annex won't try to get content from it.
So, if lockContent were implemented for web, and the web was left as the
only thing containing an object, another repo could at the same time
drop from web and remove its url, leaving no way to get the object.
Add to that, of course, the web is typically set untrusted, and so
implementing lockContent would not then be useful.
Similar reasoning applies to the bittorrent special remote, as well
as the fact that it does not even implement checkKey.
