Do not rely on git internals, support Git >= 2.10

Do not rely on git internals, support Git >= 2.10

Fast-export has traditionally sourced the internal git-sh-setup from
Git, following the release of Git 2.10 this no longer works. Fast-export
only uses the functionality of git-sh-setup for two things: cd:ing to
the git repo dir and setting up the GIT_REPO environment variable. To
future-proof fast-export start doing what we need by hand in
fast-export.

Acknowledgments to Louis Sautier who reported the problem and tested the
fix.