merge-recursive: distinguish "removed" and "overwritten" messages

merge-recursive: distinguish "removed" and "overwritten" messages

To limit the number of possible error messages, the error messages for
the case would_lose_untracked_file and would_lose_orphaned in
unpack_trees_options.msgs were handled with a single string,
parameterized by an action string ("overwritten" or "removed").

Instead, we consider them as two different cases, with unparameterized
string. This will make it easier to make separate lists sorted by error
types later.

Only the bind_overlap case still takes two %s parameters, but that's
unavoidable.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>