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