merge-recursive: Avoid unnecessary file rewrites

merge-recursive: Avoid unnecessary file rewrites

Often times, a potential conflict at a path is resolved by merge-recursive
by using the content that was already present at that location.  In such
cases, we do not want to overwrite the content that is already present, as
that could trigger unnecessary recompilations.  One of the patches earlier
in this series ("merge-recursive: When we detect we can skip an update,
actually skip it") fixed the cases that involved content merges, but there
were a few other cases as well.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>