Added tests for the case of merged and unmerged entries for the same …

…file

read-cache.c: Ensure unmerged entries are removed

Wrong implementations of tools that modify the index can left
some files as merged and unmerged at the same time. Avoid undesiderable
behaviours by handling this situation.

Signed-off-by: Jaime Soriano Pastor <[email protected]>

Git 2.1-rc2

Git 2.1.0-rc1

Git 2.0.4

Git 2.1.0-rc0

Git 2.0.3

Git 2.0.2

Git 2.0.1

Git 1.9.4