Remove the requirement for `dist-git-merge` when running `dist-git-source` and external repo

[LecrisUT]

The context here is the following plan in fmf that is trying to pick the appropriate sources of tmt to run the integration tests

summary: Run integration tests with tmt
discover:
    how: fmf
    filter: 'tier: 1 & tag:-provision-only'
    dist-git-source: true
#    dist-git-merge: true
    url: https://src.fedoraproject.org/rpms/tmt.git
    ref: $@{dist-git-branch}
execute:
  how: tmt

The interface there should git clone the dist-git of tmt for the appropriate branch we are on, extract the git dist-git-source and go into the fmf root of that.

But we currently require the dist-git-merge which tells it to combine fmf's dist-git with tmt's, which is not really what we want to do there.

CC @lukaszachy iirc you included the restriction there, does relaxing it make sense?

[lukaszachy]

IIRC it was necessary because at that time upstream tarball didn't include .fmf/version. I see it in tmt-1.72.0.tar.gz so AFAIK there is no need to keep dist-git-merge: true there anymore.

[LecrisUT]

Well, keeping it as opt-in would always be available, just that forcing it was weird. Thanks a lot for the context 🙏