[bug-diffutils] FYI, pushed a few historical tags

[Jim Meyering]

FYI, I've just tagged the following:

    v2.7
    v2.8
    v2.8.3
    v2.8.4
    v2.8.6

Determining which commit corresponded to a release was
not trivial, and for some I made the effort and examined
the corresponding tarball.  For a couple, I simply tagged
what appeared to be the latest-for-that-version commit.

There are many more test releases that remain untagged,
but for now, the above should suffice.