stage tests: ensure we can stage conflicts
stage tests: ensure we can stage conflicts

A file should be ignored when it does not exist in the repository,
but conflicts should absolutely qualify as "existing in the
repository".  Ensure that we can stage ignored files correctly.

dlsteuer/libgit2sharp,OidaTiftla/libgit2sharp,shana/libgit2sharp,xoofx/libgit2sharp,libgit2/libgit2sharp,AArnott/libgit2sharp,rcorre/libgit2sharp,ethomson/libgit2sharp,whoisj/libgit2sharp,github/libgit2sharp,Zoxive/libgit2sharp,jorgeamado/libgit2sharp,shana/libgit2sharp,sushihangover/libgit2sharp,rcorre/libgit2sharp,AArnott/libgit2sharp,oliver-feng/libgit2sharp,jeffhostetler/public_libgit2sharp,xoofx/libgit2sharp,red-gate/libgit2sharp,vorou/libgit2sharp,oliver-feng/libgit2sharp,AMSadek/libgit2sharp,Skybladev2/libgit2sharp,psawey/libgit2sharp,psawey/libgit2sharp,AMSadek/libgit2sharp,OidaTiftla/libgit2sharp,whoisj/libgit2sharp,Zoxive/libgit2sharp,jeffhostetler/public_libgit2sharp,dlsteuer/libgit2sharp,vorou/libgit2sharp,ethomson/libgit2sharp,github/libgit2sharp,sushihangover/libgit2sharp,Skybladev2/libgit2sharp,jorgeamado/libgit2sharp,red-gate/libgit2sharp,PKRoma/libgit2sharp