git-apply: try threeway first when "--3way" is used

git-apply: try threeway first when "--3way" is used

The apply_fragments() method of "git apply"
can silently apply patches incorrectly if
a file has repeating contents. In these
cases a three-way merge is capable of applying
it correctly in more situations, and will
show a conflict rather than applying it
incorrectly. However, because the patches
apply "successfully" using apply_fragments(),
git will never fall back to the merge, even
if the "--3way" flag is used, and the user has
no way to ensure correctness by forcing the
three-way merge method.

Change the behavior so that when "--3way" is used,
git will always try the three-way merge first and
will only fall back to apply_fragments() in cases
where blobs are not available or some other error
(but not in the case of a merge conflict).

Since user-facing results will be different,
this has backwards compatibility implications
for users depending on the old behavior. In
addition, the three-way merge will be slower
than direct patch application.

Signed-off-by: Jerry Zhang <jerry@skydio.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>