checkout_entry(): use the strbuf throughout the function

checkout_entry(): use the strbuf throughout the function

There is no need to break out the "buf" and "len" members into
separate temporary variables.  Rename path_buf to path and use
path.buf and path.len directly.  This makes it easier to reason about
the data flow in the function.

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