[travis] Make the git_checkout function more reliable.
[travis] Make the git_checkout function more reliable.

This commit also documents the behavior of said function;
and fix the location of the ssreflect clone to an absolute
path (this is now necessary).

Zimmi48/coq,JasonGross/coq,amintimany/coq,maximedenes/coq,william-lawvere/coq,william-lawvere/coq,SkySkimmer/coq,Zimmi48/coq,Matafou/coq,silene/coq,Ptival/coq,amintimany/coq,Ptival/coq,coq/coq,Lionel-Rieg/coq,Lionel-Rieg/coq,silene/coq,william-lawvere/coq,coq/coq,JasonGross/coq,ejgallego/coq,gares/coq,Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq,gares/coq,JasonGross/coq,letouzey/coq-wip,Lionel-Rieg/coq,silene/coq,SkySkimmer/coq,maximedenes/coq,letouzey/coq-wip,gares/coq,ppedrot/coq,ppedrot/coq,SkySkimmer/coq,coq/coq,letouzey/coq-wip,maximedenes/coq,Zimmi48/coq,ppedrot/coq,coq/coq,Ptival/coq,JasonGross/coq,maximedenes/coq,maximedenes/coq,Matafou/coq,gares/coq,letouzey/coq-wip,ppedrot/coq,ppedrot/coq,ejgallego/coq,gares/coq,amintimany/coq,letouzey/coq-wip,Matafou/coq