improve merge-pr script
improve merge-pr script

The script now performs many more checks and reports errors in
a more intelligible way.

ppedrot/coq,maximedenes/coq,maximedenes/coq,maximedenes/coq,coq/coq,ppedrot/coq,Matafou/coq,ejgallego/coq,letouzey/coq-wip,JasonGross/coq,gares/coq,gares/coq,letouzey/coq-wip,JasonGross/coq,Matafou/coq,maximedenes/coq,Zimmi48/coq,ppedrot/coq,ejgallego/coq,gares/coq,maximedenes/coq,Zimmi48/coq,silene/coq,coq/coq,silene/coq,SkySkimmer/coq,coq/coq,ejgallego/coq,ppedrot/coq,coq/coq,Zimmi48/coq,gares/coq,JasonGross/coq,silene/coq,ppedrot/coq,Matafou/coq,letouzey/coq-wip,JasonGross/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,Zimmi48/coq,SkySkimmer/coq,letouzey/coq-wip,gares/coq,letouzey/coq-wip