[BUG?] fetch into shallow sends a large number of objects

[Jeff King <peff@peff.net>]

I came across an interesting shallow-fetch case, and I suspect git is
doing something very sub-optimal.

You can reproduce with:

	git clone --bare git://github.com/CocoaPods/Specs
	cd Specs.git

	time git pack-objects --revs --thin --stdout \
	  --delta-base-offset --include-tag <<\EOF | wc -c
	d7a6d9295d718c6015be496880f1a293bdd89185
	--not
	067f265bb512c95b22b83ccd121b9facbddcf6b1
	EOF

	time git pack-objects --revs --thin --stdout --shallow \
	  --delta-base-offset --include-tag <<\EOF | wc -c
	--shallow ecd7ea6033fe8a05d5c21f3a54355fded6942659
	d7a6d9295d718c6015be496880f1a293bdd89185
	--not
	067f265bb512c95b22b83ccd121b9facbddcf6b1
	EOF

The first is a non-shallow clone; it takes a few hundred milliseconds to
generate the pack, and the result is a few hundred kilobytes. The second
is a shallow clone (logged from a real-world request); it sends 200
times as many objects, totaling 270MB, and takes almost a minute of CPU.

I'm trying to figure out why that is, and whether git can do better.

I think what's happening here is that the history looks like this:

  F ... ecd7ea6 ... 067f265 ... M ... d7a6d929
   \                           /
    X ....................... Y

That is, we are asking for 067f265 to d7a6d929, but that includes some
merge M which _crosses_ our grafted shallow-point ecd7ea6. That pulls in
essentially all of the history for the entire (missing only the commits
from the fork point F up to ecd7ea6).

So I _think_ that pack-objects is doing the best it can with the
information it was given. But presumably in a shallow repo the user
would prefer to have a segmented history rather than pull in all of
those old commits.

I don't know how the client invoked git, but we can guess what happened
and simulate with:

  git tag shallow ecd7ea6033fe8a05d5c21f3a54355fded6942659
  git tag old 067f265bb512c95b22b83ccd121b9facbddcf6b1
  git tag new d7a6d9295d718c6015be496880f1a293bdd89185

  git clone --no-local --bare --branch=shallow --depth=1 . clone.git
  cd clone.git
  git fetch origin old:refs/tags/old
  git fetch origin new:refs/tags/new

Of the two follow-up fetches in the clone, the first is reasonably fast
(it just grabs a few new commits on top of the shallow base), but the
second is expensive (it grabs the merge which pulls in the whole
history). If we add "--depth=1" to each of those fetches, everything
remains fast.

Is this user error to call "git fetch" without "--depth" in the
subsequent cases? Or should git remember that we are in a shallow repo,
and presume that the user by default wants to keep things shallow?

-Peff