Git recently added a new field to the credential helper, `wwwauth[]`,
which may be repeated and includes all of the `WWW-Authenticate` headers
so that the credential helper may choose an appropriate set of
credentials and extract any sort of necessary data from the field (such
as challenge).
In Git LFS, we also want to do this with the `LFS-Authenticate` headers
as well, since those are often used for the same purpose, so include
both these headers in that field when passing them to `git credential
fill`.
Note that `git credential fill` only honours this value and passes it to
the credential helper in Git 2.41 and newer (including the latest
`HEAD`). However, just to be safe, let's add an undocumented and
experimental option (`credential.*.skipwwwauth`) that users can use to
control this, which we can remove in a few releases if it turns out it's
not needed. Similarly, skip our new tests if we have an older version
of Git where this doesn't work, since they'll otherwise fail.
badde878a3