tests/test-remotefilelog-keepset.t

rust-changelog: remove special parsing of empty changelog data for null rev

For the null revision, `Revlog::get_rev_data()` will return an empty
string (of bytes). We currently handle that case in
`ChangelogRevisionData::manifest_node()`. However, it's going to be
ugly to have special handling for the null revision for each future
method on `ChangelogRevisionData`. This patch therefore restructures
the code so we instead initialize the struct with valid data for the
null revision.

Differential Revision: https://phab.mercurial-scm.org/D12438

#require no-windows

  $ . "$TESTDIR/remotefilelog-library.sh"

  $ hg init master
  $ cd master
  $ cat >> .hg/hgrc <<EOF
  > [remotefilelog]
  > server=True
  > serverexpiration=-1
  > EOF
  $ echo x > x
  $ hg commit -qAm x
  $ echo y > y
  $ hg commit -qAm y
  $ echo z > z
  $ hg commit -qAm z
  $ cd ..

  $ hgcloneshallow ssh://user@dummy/master shallow -q
  3 files fetched over 1 fetches - (3 misses, 0.00% hit ratio) over *s (glob)

# Compute keepset for 0th and 2nd commit, which implies that we do not process
# the 1st commit, therefore we diff 2nd manifest with the 0th manifest and
# populate the keepkeys from the diff
  $ cd shallow
  $ cat >> .hg/hgrc <<EOF
  > [remotefilelog]
  > pullprefetch=0+2
  > EOF
  $ hg debugkeepset

# Compute keepset for all commits, which implies that we only process deltas of
# manifests of commits 1 and 2 and therefore populate the keepkeys from deltas
  $ cat >> .hg/hgrc <<EOF
  > [remotefilelog]
  > pullprefetch=all()
  > EOF
  $ hg debugkeepset