.arcconfig

sparse: add test showing `hg merge` is broken while using sparse extension

This patch adds a test to demonstrate that `hg merge` is broken in some cases
while using sparse extension.

The case is when you have a file which is:

  * modified between current wdir parent and merge base
  * deleted between merge base and merge destination
  * excluded from sparse checkout

Doing `hg merge` results in file not found error. Next patch will fix this.

Differential Revision: https://phab.mercurial-scm.org/D3983

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