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
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.