[team-infra] add standardized `git st` alias

This adds a git alias which can be shared by all infrastructure team
members; `git st` which will show the current git status in short
form, but also displaying branch info.

PS: feel free to adopt this more widely; I just figure it's easier to
get buy-in gradually.

cc @dcarley @nickstenning @samjsharpe @rjw1 @singhgarima @danielabel