tests/test-rebase-legacy.t

internals: document CBOR utilization

I spoke with some people at Mozilla about CBOR and they advised me
that we should be careful about the subset of CBOR we use in order
to mitigate security, performance, and compatibility concerns.

This commit establishes a document that attempts to formalize our
use of CBOR.

Its main limitations are on what types are allowed. It explicitly
enumerates which types are supported. Notable missing features
include:

* Indefinite-length arrays and maps
* Text strings (bytes all the way)
* Floats
* Date/time types
* Big integers
* Use of indefinite-length byte strings for map keys, values in
  containers.

If we have a need for any of these, we can have a discussion about
them when the time comes.

Differential Revision: https://phab.mercurial-scm.org/D4412

Test rebase --continue with rebasestate written by legacy client

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > rebase=
  > drawdag=$TESTDIR/drawdag.py
  > EOF

  $ hg init
  $ hg debugdrawdag <<'EOF'
  >    D H
  >    | |
  >    C G
  >    | |
  >    B F
  >    | |
  >  Z A E
  >   \|/
  >    R
  > EOF

rebasestate generated by a legacy client running "hg rebase -r B+D+E+G+H -d Z"

  $ touch .hg/last-message.txt
  $ cat > .hg/rebasestate <<EOF
  > 0000000000000000000000000000000000000000
  > f424eb6a8c01c4a0c0fba9f863f79b3eb5b4b69f
  > 0000000000000000000000000000000000000000
  > 0
  > 0
  > 0
  > 
  > 21a6c45028857f500f56ae84fbf40689c429305b:-2
  > de008c61a447fcfd93f808ef527d933a84048ce7:0000000000000000000000000000000000000000
  > c1e6b162678d07d0b204e5c8267d51b4e03b633c:0000000000000000000000000000000000000000
  > aeba276fcb7df8e10153a07ee728d5540693f5aa:-3
  > bd5548558fcf354d37613005737a143871bf3723:-3
  > d2fa1c02b2401b0e32867f26cce50818a4bd796a:0000000000000000000000000000000000000000
  > 6f7a236de6852570cd54649ab62b1012bb78abc8:0000000000000000000000000000000000000000
  > 6582e6951a9c48c236f746f186378e36f59f4928:0000000000000000000000000000000000000000
  > EOF

  $ hg rebase --continue
  rebasing 4:c1e6b162678d "B" (B)
  rebasing 8:6f7a236de685 "D" (D)
  rebasing 2:de008c61a447 "E" (E)
  rebasing 7:d2fa1c02b240 "G" (G)
  rebasing 9:6582e6951a9c "H" (H tip)
  warning: orphaned descendants detected, not stripping c1e6b162678d, de008c61a447
  saved backup bundle to $TESTTMP/.hg/strip-backup/6f7a236de685-9880a3dc-rebase.hg

  $ hg log -G -T '{rev}:{node|short} {desc}\n'
  o  11:721b8da0a708 H
  |
  o  10:9d65695ec3c2 G
  |
  o  9:21c8397a5d68 E
  |
  | o  8:fc52970345e8 D
  | |
  | o  7:eac96551b107 B
  |/
  | o  6:bd5548558fcf C
  | |
  | | o  5:aeba276fcb7d F
  | | |
  | o |  4:c1e6b162678d B
  | | |
  o | |  3:f424eb6a8c01 Z
  | | |
  +---o  2:de008c61a447 E
  | |
  | o  1:21a6c4502885 A
  |/
  o  0:b41ce7760717 R