remote-curl: fix large pushes with GSSAPI

remote-curl: fix large pushes with GSSAPI

Due to an interaction between the way libcurl handles GSSAPI
authentication over HTTP and the way git uses libcurl, large
pushes (those over http.postBuffer bytes) would fail due to
an authentication failure requiring a rewind of the curl
buffer.  Such a rewind was not possible because the data did
not fit into the entire buffer.

Enable the use of the Expect: 100-continue header for large
requests where the server offers GSSAPI authentication to
avoid this issue, since the request would otherwise fail.
This allows git to get the authentication data right before
sending the pack contents.  Existing cases where pushes
would succeed, including small requests using GSSAPI, still
disable the use of 100 Continue, as it causes problems for
some remote HTTP implementations (servers and proxies).

Signed-off-by: Brian M. Carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>