Add 'pulls compare branch1 branch2' showing only the merge PR commits that are in branch1 that are not in branch2

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>
Run pull request filters concurrently

Before: pulls --mine  2.26s user 1.33s system 18% cpu 19.892 total
After: pulls --mine  2.30s user 1.06s system 64% cpu 5.239 total

Added a sorter to make sure the output is actually sorted (like it was
before). Maybe this should move elsewhere?
Merge pull request #156 from cpuguy83/run_pull_filters_concurrently

Run pull request filters concurrently
Merge pull request #154 from SvenDowideit/add-pr-compare

Add 'pulls compare branch1 branch2' showing only the merge PR commits th...
Cleans up dup checks on maintainer || mine