subtree: add comments and sanity checks

subtree: add comments and sanity checks

For each function in subtree, add a usage comment saying what the
arguments are, and add an `assert` checking the number of arguments.

In figuring out each thing's arguments in order to write those comments
and assertions, it turns out that find_existing_splits is written as if
it takes multiple 'revs', but it is in fact only ever passed a single
'rev':

	unrevs="$(find_existing_splits "$dir" "$rev")" || exit $?

So go ahead and codify that by documenting and asserting that it takes
exactly two arguments, one dir and one rev.

Signed-off-by: Luke Shumaker <lukeshu@datawire.io>
Signed-off-by: Junio C Hamano <gitster@pobox.com>