CommitService.FindCommits: disabling `follow` for Git Range Notations

Git range revspecs have the potential to be as much harmful
with the `follow` option than the redundant ancestors iteration
we already removed for #117, but it will take us more time
to find out how to treat them efficiently.

Meanwhile, we prefer to fall back to ignoring copy-tracing rather
than risking it to crash the entire system. In Git world, copy-tracing
is a best-effort anyway (heuristic with several levels of analysis)

This finishes the work on this nasty resource exhaustion issue.

Closes #117
2 jobs for !126 with topic/stable/find-commits-explosion in 4 minutes and 21 seconds (queued for 2 seconds)
latest merge request