Perf bug: rev-list w/ 2+ paths relatively slow with commit-graph

Reproduce steps: ``` git clone https://github.com/golang/go.git cd go git config core.commitGraph true git commit-graph write --split --reachable --changed-paths # Without this, all calls equally slow (~1s) time git rev-list -10 3730814f2f2bf24550920c39a16841583de2dac1 -- src/clean.bash > /dev/null # ~90ms time git rev-list -10 3730814f2f2bf24550920c39a16841583de2dac1 -- src/Make.dist > /dev/null # ~100ms time git rev-list -10 3730814f2f2bf24550920c39a16841583de2dac1 -- src/clean.bash src/Make.dist > /dev/null # ~650ms ```

The rev-list call with multiple paths takes over 3x longer than the sum of individual calls to it for the same files.

Expectation: rev-list with multiple paths should take <= the sum of
the time it takes to call it with each path individually (ideally <,
since with the count limit it should be able to early-exit and search
less commits for either path).

Also reproduces without the -10 arg, or with a lower count (double instead of triple w/ -1), but these results are perhaps most surprising with a count present.