Document requirement to have git >= 2.7 to use the merge script.
Document requirement to have git >= 2.7 to use the merge script.

As reported in https://github.com/coq/coq/issues/7097#issuecomment-378632415

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