We were trying to 'git checkout $prefix', which is ambiguous if $prefix
names a directory *and* a branch. Do 'git checkout -- $prefix' instead.
The main place this appeared was in 'git subtree add'.
Reported by several people.
Now we only prune out a commit if it has exactly one remaining parent and
that parent's tree is identical to ours.
But I also changed the test to create the initial "-s ours" merge in one
step instead of two, and that merge can be eliminated since one of its
parents doesn't affect the subdir at all, and is thus deleted.
Now we cut out a commit if any of its parents had the same tree; just use
that parent in its place. This makes the history look nice, but I don't
think it's quite right...
