git-read-tree --index-output=<file>

git-read-tree --index-output=<file>

This corrects the interface mistake of the previous one, and
gives a command line parameter to the only plumbing command that
currently needs it: "git-read-tree".

We can add the calls to set_alternate_index_output() to other
plumbing commands that update the index if/when needed.

Signed-off-by: Junio C Hamano <redacted>