Support a merge with conflicting gitlink change

Support a merge with conflicting gitlink change

merge-recursive did not support merging trees that have conflicting
changes in submodules they contain, and died.  Support it exactly the
same way as how it handles conflicting symbolic link changes --- mark it
as a conflict, take the tentative result from the current side, and
letting the caller resolve the conflict, without dying in merge_file()
function.

Also reword the error message issued when merge_file() has to die
because it sees a tree entry of type it does not support yet.

[jc: fixed up initial draft by Finn Arne Gangstad]

Signed-off-by: Junio C Hamano <gitster@pobox.com>