Make pull() take some implicit data as explicit arguments

Make pull() take some implicit data as explicit arguments

Currently it's a bit weird that pull() takes a single argument
describing the commit but takes the write_ref from a global variable.
This makes it take that as a parameter as well, which might be nicer
for the libification in the future, but especially it will make for
nicer code when we implement pull()ing multiple commits at once.

Signed-off-by: Petr Baudis <redacted>
Signed-off-by: Junio C Hamano <redacted>