Fix git recognition when in worktrees.
Fix git recognition when in worktrees.

git worktrees have a .git file instead of a .git directory.
Using Sys.file_exists is a more general solution which gives true in both cases.

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