fix(shell/lean.cpp): print git commit in version string only when it's available
fix(shell/lean.cpp): print git commit in version string only when it's available

soonhokong/lean-osx,eigengrau/lean,leanprover/lean,soonhokong/lean,leodemoura/lean,c-cube/lean,digama0/lean,fgdorais/lean,soonhokong/lean-osx,htzh/lean,levnach/lean,avigad/lean,javra/lean,avigad/lean,avigad/lean,dselsam/lean,fpvandoorn/lean2,Kha/lean,fpvandoorn/lean2,dselsam/lean,fgdorais/lean,johoelzl/lean,leanprover-community/lean,UlrikBuchholtz/lean,htzh/lean,UlrikBuchholtz/lean,javra/lean,johoelzl/lean,javra/lean,leanprover/lean,soonhokong/lean-osx,fpvandoorn/lean,levnach/lean,dselsam/lean,soonhokong/lean,levnach/lean,dselsam/lean,soonhokong/lean-windows,soonhokong/lean,fpvandoorn/lean,leanprover-community/lean,htzh/lean,leodemoura/lean,fpvandoorn/lean,fgdorais/lean,rlewis1988/lean,soonhokong/lean-osx,avigad/lean,soonhokong/lean-osx,soonhokong/lean,leodemoura/lean,fgdorais/lean,johoelzl/lean,avigad/lean,c-cube/lean,fpvandoorn/lean2,rlewis1988/lean,UlrikBuchholtz/lean,Kha/lean,leodemoura/lean,sp3ctum/lean,leanprover/lean,leanprover/lean,htzh/lean,soonhokong/lean-windows,rlewis1988/lean,htzh/lean,johoelzl/lean,dselsam/lean,leanprover-community/lean,eigengrau/lean,Kha/lean,sp3ctum/lean,johoelzl/lean,digama0/lean,digama0/lean,rlewis1988/lean,fpvandoorn/lean2,Kha/lean,Kha/lean,rlewis1988/lean,UlrikBuchholtz/lean,digama0/lean,levnach/lean,leanprover-community/lean,leanprover-community/lean,soonhokong/lean,digama0/lean,eigengrau/lean,sp3ctum/lean,c-cube/lean,soonhokong/lean-windows,eigengrau/lean,sp3ctum/lean,UlrikBuchholtz/lean,levnach/lean,digama0/lean,fgdorais/lean,dselsam/lean,fpvandoorn/lean,leanprover-community/lean,soonhokong/lean-windows,c-cube/lean,javra/lean,leanprover/lean,leodemoura/lean,avigad/lean,eigengrau/lean,johoelzl/lean,c-cube/lean,javra/lean,rlewis1988/lean,leanprover/lean,fpvandoorn/lean,leodemoura/lean,Kha/lean,fpvandoorn/lean2,sp3ctum/lean,soonhokong/lean-windows,fpvandoorn/lean,fgdorais/lean