Install ripgrep to search against conflicts in the source
I hate installing packages like this, but it doesn't seem that
I could just apt-get install cargo
for this image. It might be
a bit too old to have that.
Whenever it's available, we should just install from apt-get.
Note that silversearcher-ag isn't working as good as ripgrep, because it's slower and it doesn't show the filename which contains the conflicts without tty. It might be a bug, because having tty would work as expected.
See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13141