Let the one-rev check pass a lockfile with no git sources

[pathscale]

one-rev is red on master. It is not a dependency problem: the check itself
fails when there is nothing to check.

grep exits 1 when it matches nothing, and the script runs under
set -euo pipefail, so an empty match fails the whole script. Taking the last
git dependency out of Cargo.lock (#25) is therefore what broke it. The script
that exists to police git dependencies now fails on the best possible lockfile.

The success message was also unconditional — it printed "one rev per git
source" whether or not any git source existed, which reads as a pass of a check
that never ran. It says "no git sources" in that case now.

Verified

lockfile	before	after
no git sources (current master)	exit 1	exit 0, "no git sources"
one rev per source	exit 0	exit 0, "one rev per git source"
same repo at two revs	exit 1	exit 1, unchanged message

The duplicate-detection path is untouched; only the empty case and the final
message changed.