chore: repoint vendor/zoekt at upstream-ancestry merge commit

sourcebot-dev/zoekt#10 was squash-merged into zoekt@main, which
flattened the merge commit and left GitHub reporting the fork as 108
commits behind sourcegraph/zoekt:main even though all upstream content
was present. Fixed by performing a 'git merge -s ours upstream/main'
on zoekt@main: this records upstream/main as a second parent without
changing any files, restoring the ancestry link.

Bumping this submodule pointer from 7c6c629f (the previous main tip)
to df983ea1 (the new merge-ours commit). The vendored tree content is
byte-identical to 7c6c629f; only the commit graph is different.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: msukkari