tests/test-merge4.t

resourceutil: don't limit resources to the `mercurial` package

This should make things a little clearer, in that it now requires the full
package name to access a resource.  But the real motivation is that
`extensions._disabledpaths()` walks the `hgext` directory looking for bundled
extensions.  This in turn feeds, among other things:

    1) Listing disabled extensions in `hg help extensions`
    2) Indicating that an unknown command is in a non-enabled extension
    3) Displaying help for non-enabled extensions
    4) Generating documentation
    5) Announcing LFS is auto-enabled (or not) when cloning from an LFS source

The filesystem based ResourceReader will happily return *.py and *.pyc, but the
one supplied by PyOxidizer doesn't.  Presumably we can change that.  The only
other idea I had here is for setup.py to generate a text file containing the
list of extensions, but that doesn't seem great when running from source.

Differential Revision: https://phab.mercurial-scm.org/D7772

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg update 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo This is file c1 > c
  $ hg add c
  $ hg commit -m "commit #2"
  created new head
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ rm b
  $ echo This is file c22 > c

Test hg behaves when committing with a missing file added by a merge

  $ hg commit -m "commit #3"
  abort: cannot commit merge with missing files
  [255]