checkout-index: plug memory leak from prefix_path()
checkout-index: plug memory leak from prefix_path()

prefix_path() sometimes allocates new memory and returns it, and
other times returns the incoming path argument intact.  The
callers need to be a bit careful not to leak memory.

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