refspec: replace `refspec_init()` with fetch/push variants

refspec: replace `refspec_init()` with fetch/push variants

To avoid having a Boolean argument in the refspec_init() function,
replace it with two variants:

  - `refspec_init_fetch()`
  - `refspec_init_push()`

to codify the meaning of that Boolean into the function's name itself.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>