read-tree: shadowed variable fix.
read-tree: shadowed variable fix.

Recent changes to built-ins broke committing from subdirectory,
because the unused parameter "prefix" shadowed a global variable.

Spotted by Jeff King.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git