[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.
Allow teams::infrastructure users to disable git config

@nickstenning pointed out that some people manage their ~/.gitconfig
directory themselves and so a git::config::global resource will
interact with that file resource in bad ways.

This commit adds a parameter to allow a user of teams::infrastructure
to disable its management of git config.
Merge pull request #18 from alphagov/git_alias_st_infrastructure

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