git-fetch: rewrite another shell loop in C

git-fetch: rewrite another shell loop in C

Move another shell loop that canonicalizes the list of refs for
underlying git-fetch-pack and fetch-native-store into C.

This seems to shave the runtime for the same 1000 branch
repository from 30 seconds down to 15 seconds (it used to be 2
and half minutes with the original version).

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