I'm going to add changed-path index, and the operation link file will store a
list of segment files and a starting commit position. Suppose the link file is
small, we wouldn't need our own serialization format.
This patch adds new directory for proto-based operation link files. We could
reuse the existing directory, but that would make debugging a bit harder.
It was convenient that what the git backend stored in its "extras"
table is exactly a subset of the fields that local backend stores, but
it's bit ugly and limiting. For example, it makes it possible to
populate the `author` field in the git extras, but that would have no
effect. It's better that it's not possible to do that (we store the
author field in the git commit, of course).
What made me notice this now was that I'm working on tree-level
conflicts (#1624) and I'm thinking of adding a field to the git extras
saying "this commit has single tree, but it's still a new-style
commit", so we can know not to walking such trees to find path-level
conflicts. That's only needed for the git backend because we don't
care about compatibility for the local backend.
