lib/protocol: Revert protobuf encoder changes in v0.14.17 (fixes #3855)

The protobuf encoder now produces packed arrays for things like []int32,
which is actually correct according to the proto3 spec. However
Syncthing v0.14.16 and earlier doesn't support this. This reverts the
encoding change, but keeps the updated decoder so that we are both more
compatible with other proto3 implementations and can move to the updated
encoder in the future.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3856

.gitattributes

@@ -6,4 +6,3 @@ vendor/**   -text=auto
 
 # Diffs on these files are meaningless
 *.svg           -diff
-*.pb.go         -diff