ConnectionExists: re-use connections better

When allowing NTLM, the re-use connection logic was too focused on
finding an existing NTLM connection to use and didn't properly allow
re-use of other ones. This made the logic not re-use perfectly re-usable
connections.

Added test case 1418 and 1419 to verify.

Regression brought in 8ae35102 (curl 7.35.0)

Reported-by: Jeff King
Bug: http://thread.gmane.org/gmane.comp.version-control.git/242213