[merge] InterKnit.join() doesn't need the complete revision graph

[John Arbash Meinel]

I'm still doing some profiling, and I found another surprise. When
pushing a small branch into another small branch, but using a local and
remote repository with a lot of revisions, InterKnit.join() touches too
many revisions.

     4  5.6062  0.1274   bzrlib.knit:1560(join)
+21453  0.2398  0.1607   +bzrlib.knit:509(has_version)
    +4  0.4071  0.1089   +bzrlib.versionedfile:307(get_graph)
    +4  1.6765  0.0048   +bzrlib.tsort:27(topo_sort)
  +152  0.0017  0.0011   +bzrlib.knit:872(get_parents_with_ghosts)
    +8  0.0036  0.0002   +bzrlib.knit:1454(read_records_iter_raw)
    +4  3.1369  0.0002   +bzrlib.knit:384(_add_raw_records)

The actual plugin I'm pushing only has 25 revisions, as would the
target. I don't need to check all 9181 revisions in my repository.
(21453 is because you have inventory.knit, revisions.knit,
signatures.knit, and the text.knit, where signatures and text have a
very different number of revisions).

The attached patch just culls out the unnecessary revisions ahead of
time. I haven't profiled it a lot to see how much it helps for cases
like this. But it seems it should cut down the topo_sort time, if not
the get_graph time.

Also, would it be reasonable to use merge_sort() since that seems to be
faster than topo_sort?

John
=:->
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