Fix arc diff in SVN with paths

Summary:
Recently, in D4097 or one of the precursors I refactored this. However, when $rev is null parseBaseCommitArgument() throws ("This VCS does not support commit ranges."). Shield the call so it only happens if if $rev is nonempty (we still want to make the call, so "arc lint --rev x" on SVN will throw and inform the user that "--rev" is incorrect usage).

(@vrana, this was reported by FB and might be worth pushing.)

Test Plan: Ran "arc diff --preview <path>". Grepped for other parseBaseCommitArgument() callsites and verified they don't have similar issues.

Reviewers: vrana, btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4241