credential.c: store "wwwauth[]" values in `credential_read()`

credential.c: store "wwwauth[]" values in `credential_read()`

Teach git-credential to read "wwwauth[]" value(s) when parsing the
output of a credential helper.

These extra headers are not needed for Git's own HTTP support to use the
feature internally, but the feature would not be available for a
scripted caller (say, git-remote-mediawiki providing the header in the
same way).

As a bonus, this also makes it easier to use wwwauth[] in synthetic
credential inputs in our test suite.

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>