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 <junkio@cox.net>

git-commit.sh

@@ -370,8 +370,8 @@ t,)
 		# the same way.
 		if test -z "$initial_commit"
 		then
-			_GIT_INDEX_OUTPUT="$TMP_INDEX" \
-			GIT_INDEX_FILE="$THIS_INDEX" git-read-tree -i -m HEAD
+			GIT_INDEX_FILE="$THIS_INDEX" \
+			git-read-tree --index-output="$TMP_INDEX" -i -m HEAD
 		else
 			rm -f "$TMP_INDEX"
 		fi || exit