tests/test-diff-issue2761.t

storageutil: extract filelog.cmp() to a standalone function

As part of implementing an alternate storage backend, I found myself
reimplementing this code.

With a little massaging, we can extract filelog.cmp() to a standalone
function.

As part of this, the call to revlog.cmp() was inlined (it is just a
2-line function).

I also tweaked some variable names to improve readability. I'll
further tweak names in a subsequent commit.

Differential Revision: https://phab.mercurial-scm.org/D4801

Test issue2761

  $ hg init

  $ touch to-be-deleted
  $ hg add
  adding to-be-deleted
  $ hg ci -m first
  $ echo a > to-be-deleted
  $ hg ci -m second
  $ rm to-be-deleted
  $ hg diff -r 0

Same issue, different code path

  $ hg up -C
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ touch does-not-exist-in-1
  $ hg add
  adding does-not-exist-in-1
  $ hg ci -m third
  $ rm does-not-exist-in-1
  $ hg diff -r 1