fetch: allow refspecs specified through stdin

fetch: allow refspecs specified through stdin

In a subsequent patch, partial clones will be taught to fetch missing
objects using a "git fetch" subprocess. Because the number of objects
fetched may be too numerous to fit on the command line, teach "fetch" to
accept refspecs passed through stdin.

Signed-off-by: Jonathan Tan <redacted>
Signed-off-by: Junio C Hamano <redacted>