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Added fix for gitclient checkout shallow with git >= 1.7.10

Without the fix, other branches than the given one may not
get checked out, nor fetched on the next fetch.

https://raw.github.com/gitster/git/master/Documentation/RelNotes/1.7.10.txt
and
http://git-scm.com/docs/git-clone/1.7.10

    "git clone" learned "--single-branch" option to limit cloning
    to  a single branch (surprise!); tags that do not point into the
    history of the branch are not fetched.

--single-branch
    Clone only the history leading to the tip of a single branch,
    either specified by the --branch option or the primary branch
    remote's HEAD points at. When creating a shallow clone with
    the --depth option, this is the default, unless --no-single-branch
    is given to fetch the histories near the tips of all branches.

Conflicts:

	src/vcstools/git.py