The steps to update the microsoft-git cask are:
1. brew update
2. brew upgrade --cask microsoft-git
This is adapted from the UpgradeVerb within microsoft/scalar. There is
one important simplification: Scalar needed to check 'brew list --cask'
to find out if the 'scalar' cask or the 'scalar-azrepos' cask was
installed (which determined if the 'microsoft-git' cask was a necessary
dependency). We do not need that here, since we are already in the
microsoft-git cask.
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
