Pull test verifies that pulled tag objects become gone after becoming
represented via specially-crafted commits. The "gone" is verified by
removing all unreachable objects and asserting that looking up an object
by original tag sha1 results in error.

However there was a thinko in "removing all unreachable objects" - this
step was implemented via `git prune`, but as documented, `git prune`
removes only loose unreachable objects, while unreachable objects that
are already in packs remain intact. From `git prune --help`:

    Note that unreachable, packed objects will remain. If this is not desired, see git-repack(1).

So previously we were lucky that running the tests did not pull objects
from test repositories into packs and plain `git prune` was enough.
However starting from Git 2.31 fetching objects from test repositories
pulls some of them as packs and the test starts to fail because original
tag object remains alive:

    === RUN   TestPullRestore
    # creating root commit
    # building "already-have" index
    # git  b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/HEAD
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/config
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/description
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/info/exclude
    # file b0       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/0/dir/empty repo.git/refs/.keep
    # building "already-have" index
    # git  b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/HEAD
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/config
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/description
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/info/exclude
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/packed-refs
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/refs/tags/tag-to-blob
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/hello.git/refs/test/ref-to-tree
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir/world.txt
    # git  b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/HEAD
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/config
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/description
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/info/exclude
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/empty repo.git/refs/.keep
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/file 2
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/nongit.git/latest
    # git  b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/HEAD
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/config
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/description
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/info/exclude
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/dir 2 + β/repo with+fragile name %αβγ.git/refs/heads/master
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/file
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/file with space + α
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/fileexec
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/symlink.dir
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/symlink.file
    # file b1       <- /home/kirr/src/neo/src/lab.nexedi.com/kirr/git-backup/testdata/1/symlink.missing
        git-backup_test.go:203: tag f735011c9fcece41219729a33f7876cd8791f659 still present in backup.git after git-prune
    --- FAIL: TestPullRestore (0.14s)

Bisecting shows that particular behaviour of pulling into packs from test
repositories starts to happen after https://git.kernel.org/pub/scm/git/git.git/commit/?id=5476e1efded571e374cd97c7d69f17962ba1c44f .
But as explained in the beginning it is not a Git bug as in the general case just `git prune` is not enough to remove all unreachable objects.

-> Fix it by accompanying `git prune` with `git repack -ad` that removes unreachable objects from the packs.

NOTE just `git repack -ad` is not enough as it puts in packs only reachable
objects, so any unreachable objects that were loose before repacking, will
remain alive.