builtin-apply: do not declare patch is creation when we do not know it

builtin-apply: do not declare patch is creation when we do not know it

When we see no context nor deleted line in the patch, we used to declare
that the patch creates a new file.  But some people create an empty file
and then apply a patch to it.  Similarly, a patch that delete everything
is not a deletion patch either.

This commit corrects these two issues.  Together with the previous commit,
it allows a diff between an empty file and a line-ful file to be treated
as both creation patch and "add stuff to an existing empty file",
depending on the context.  A new test t4126 demonstrates the fix.

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