Retire git-export.

Retire git-export.

git-export was done as a concept example on how easy it is to export
the git data to something else. It's much less powerful than any
number of trivial one-liner scripts now, and real exporters would not
ever use git-export.

It's obviously much less powerful than "git-whatchanged", or just
about any combination of git-rev-list + git-diff-tree.

Signed-off-by: Junio C Hamano <junkio@cox.net>