Craft automatic staging area default commit

Summary:
Provide a staging-commit-message command to get a default content
for submodule update commits.

Fixes T1470.

Test Plan:

$ git commit -a -m "$(staging-commit-message)"

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1470

Differential Revision: https://devcentral.nasqueron.org/D1957