fetch: avoid quadratic loop checking for updated submodules

Recent versions of git can be slow to fetch repositories with a
large number of refs (or when they already have a large
number of refs). For example, GitHub makes pull-requests
available as refs, which can lead to a large number of
available refs. This slowness goes away when submodule
recursion is turned off:

  $ git ls-remote git://github.com/rails/rails.git | wc -l
  3034

  [this takes ~10 seconds of CPU time to complete]
  git fetch --recurse-submodules=no \
    git://github.com/rails/rails.git "refs/*:refs/*"

  [this still isn't done after 10 _minutes_ of pegging the CPU]
  git fetch \
    git://github.com/rails/rails.git "refs/*:refs/*"

You can produce a quicker and simpler test case like this:

  doit() {
    head=`git rev-parse HEAD`
    for i in `seq 1 $1`; do
      echo $head refs/heads/ref$i
    done >.git/packed-refs
    echo "==> $1"
    rm -rf dest
    git init -q --bare dest &&
      (cd dest && time git.compile fetch -q .. refs/*:refs/*)
  }

  rm -rf repo
  git init -q repo && cd repo &&
  >file && git add file && git commit -q -m one

  doit 100
  doit 200
  doit 400
  doit 800
  doit 1600
  doit 3200

Which yields timings like:

  # refs  seconds of CPU
     100            0.06
     200            0.24
     400            0.95
     800            3.39
    1600           13.66
    3200           54.09

Notice that although the number of refs doubles in each
trial, the CPU time spent quadruples.

The problem is that the submodule recursion code works
something like:

  - for each ref we fetch
    - for each commit in git rev-list $new_sha1 --not --all
      - add modified submodules to list
  - fetch any newly referenced submodules

But that means if we fetch N refs, we start N revision
walks. Worse, because we use "--all", the number of refs we
must process that constitute "--all" keeps growing, too. And
you end up doing O(N^2) ref resolutions.

Instead, this patch structures the code like this:

  - for each sha1 we already have
    - add $old_sha1 to list $old
  - for each ref we fetch
    - add $new_sha1 to list $new
  - for each commit in git rev-list $new --not $old
    - add modified submodules to list
  - fetch any newly referenced submodules

This yields timings like:

  # refs  seconds of CPU
  100               0.00
  200               0.04
  400               0.04
  800               0.10
  1600              0.21
  3200              0.39

Note that the amount of effort doubles as the number of refs
doubles. Similarly, the fetch of rails.git takes about as
much time as it does with --recurse-submodules=no.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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