Because spawning separate process per 1 commit is slow.

Libgit2 does not allow to create commits only knowing tree & parentv
sha1s, but we can create commit objects by hand pretty easily - their format is

    tree <sha1>
    parent <parent1-sha1>
    parent <parent2-sha1>
    ...
    author user <email> date +offset
    committer user <email> date +offset
    LF
    message

Time for pulling-in kirr/slapos.git

before: 2.5s
after:  0.9s

NOTE AuthorInfo is changed to inherit from git.Signature (same fields
    and semantic)

NOTE Since libgit2 default ident can fail, and does not look beyond
    user.name and user.email we do backup identity detection
    (user/hostname) - in similar way Git does - ourselves.