builtin-prune: make file-scope static struct to an argument.

builtin-prune: make file-scope static struct to an argument.

I want to make the first part of 'git prune' that marks the
reachable objects callable as a library, so this starts the
first step toward the goal by making the callchain to pass
rev_info structure as an argument.

No functionality change should be in this step.

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