read-tree: --prefix=<path>/ option.

With "--prefix=<path>/" option, read-tree keeps the current
index contents, and reads the contents of named tree-ish under
directory at `<prefix>`.  The original index file cannot have
anything at the path `<prefix>` itself, and have nothing in
`<prefix>/` directory.  This can be used to graft an
independent tree into a subdirectory of the current index.

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