t5551: pack refs after creating many tags

We have two tests that create 2,000 and 100,000 tags respectively.
After doing so, the resulting state can be a bit slow to work with when
using the "files" ref backend, as each of those refs is in its own file.

This isn't a very realistic scenario, as we'd expect most of those refs
to be packed. If they accrue over time along with objects, they'd get
packed by maintenance/gc runs. And if you have a process that creates a
ton of refs at once (like a big fast-import), the usual recommendation
is to run maintenance afterwards.

So let's follow that recommendation and pack the refs ourselves.
Unfortunately, this does not seem to produce an improvement to the
run-time of the test script! That's because after producing this state,
we perform only a few fetches of it. And packing the refs costs at least
as much as serving a ref advertisement (both have to iterate the refs,
but packing additionally must write .lock files as we pack).

My wall-clock time was slightly improved (but within the noise) with
this patch, but my user and system CPU time were slightly worse!
However, on a loaded system with I/O bottlenecks, it may be a net win.
That's somewhat of a guess, though.

It would be nice if we had a way to generate all of these refs without
writing so many individual files. But even if we taught the ref code to
write large cases directly to the packed-refs file, we'd still need to
take individual locks. The real solution is a backend like reftable,
which shaves ~30% off of the test runtime.

Signed-off-by: Jeff King <peff@peff.net>

Author: peff

t/t5551-http-fetch-smart.sh

@@ -393,6 +393,7 @@ create_tags () {
 	tag=$(perl -e "print \"bla\" x 30") &&
 	sed -e "s|^:\([^ ]*\) \(.*\)$|create refs/tags/$tag-\1 \2|" <marks >input &&
 	git update-ref --stdin <input &&
+	git pack-refs --all &&
 	rm input
 }