Built a way to make an export track changes to a branch.

	git annex export --tracking master --to myexport

That ties in nicely with `git annex sync`:

	joey@darkstar:~/tmp/bench/a> echo hello > foo
	joey@darkstar:~/tmp/bench/a> git annex add
	add foo ok
	joey@darkstar:~/tmp/bench/a> git annex sync --content
	commit  
	[master 8edbc6f] git-annex in joey@darkstar:~/tmp/bench/a
	 1 file changed, 1 insertion(+)
	 create mode 120000 foo
	ok
	export myexport foo 
	ok                          
	joey@darkstar:~/tmp/bench/a> git mv foo bar
	joey@darkstar:~/tmp/bench/a> git annex sync --content
	commit  
	[master 3ab6e73] git-annex in joey@darkstar:~/tmp/bench/a
	 1 file changed, 0 insertions(+), 0 deletions(-)
	 rename foo => bar (100%)
	ok
	rename myexport foo -> .git-annex-tmp-content-SHA256E-s6--5891b5b522d5df086d0ff0b110fbd9d21bb4fc7163af34d08286a2e846f6be03 ok
	rename myexport .git-annex-tmp-content-SHA256E-s6--5891b5b522d5df086d0ff0b110fbd9d21bb4fc7163af34d08286a2e846f6be03 -> bar ok