So far, lazygit has always auto-staged files as soon as the conflict markers
disappeared from them, which means that we could rely on any file that still had
a status of "UU" to still contain conflict markers.
We are going to make the auto-staging optional in the next commit, and in that
case the user will want to manually stage "UU" files; so we must now check
whether the file contains conflict markers, and disallow the staging in that
case.
1191aca60f