Check if pickmes are already merged into master, and if so ignore them

Checking pickmes that are already merged into master would result in a conflict-master tag, since there would be nothing to commit. 

Before conflict checking a pickme against master, check if it is in master. If so, break.
Before checking a pickme against a second pickme, check the second pickme is not in master.

To hopefully avoid running rev-list more than is necessary, do very basic caching of SHAs that are known to be in master.
It doesn't load the entire master branch, only branches that it has been called to verify. 
A quick estimate on 195036 commits at 40 bytes per sha would be ~8Mb (not counting python overhead), so we could theoretically just load every sha, which would be faster than querying git