keepalive: fix merging upstream

When upstream truly diverged from the downstream repository, a merge
base needs to be determined, otherwise the merge will fail with:

  fatal: refusing to merge unrelated histories

Unfortunately, merge base calculation will also fail when one of the
branches is _shallow_, in which case the error message is actually
highly misleading.

Let's just ensure that we do not have a shallow clone when running in a
fork. There would be an opportunity to make this logic smarter, by using
the GitHub REST API to check whether there _are_ upstream commits that
aren't reachable from the downstream HEAD, but https://xkcd.com/1205/...

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

.github/workflows/keepalive.yml

@@ -21,6 +21,7 @@ jobs:
         id: merge
         if: github.event.repository.fork == true
         run: |
+          git fetch --unshallow origin &&
           git fetch https://github.com/gitgitgadget/gitgitgadget-workflows HEAD &&
           if test 0 = $(git rev-list --count HEAD..FETCH_HEAD)
           then