Notify user if fresh upstream commits are available

This patch adds a script to profile.d/ which notifies the user at the start of
an interactive shell if there are upstream MediaWiki-Vagrant commits that have
not been merged to the local master branch. The patch complements I4a6529f30,
which ensures that remote commits are fetched periodically.

Change-Id: I2bc14934ff4c607eec0c9a31ae24820d7742c08f