Bumped maximal number of pipelines created by a single push

It could also be a single transaction initied from the server side.
The limit itself is there for protection: before it landed, it was
possible to import a project with 1000 tags and then run 1000
pipelines.

There's no strong rationale upstream for so low a limit, except
that usually, people don't push more than a couple branches.

See #270 for links to upstream MRs.

On the Mercurial side however, it's easy to rebase a bunch of
stacked topics in one go, and it is very surprising to users
if it doesn't lead to as many pipelines. In the absolute, I'd prefer
at least some way to explain why all the pipelines weren't launch
and provide an easy way to do so (hence avoiding that validation
of $N$ Merge request would go trough O(N^2) pipelines), but in the
meanwhile bumping the value is good enough.

Closes: #270

--HG--
branch : heptapod
8 jobs for !177 with topic/heptapod/pipeline-limit in 17 minutes and 50 seconds (queued for 3 seconds)
merge request