Improve commit message when updating subscribed git submodules

When a git submodule is updated in a project, the update is done with
a commit that has the title "Updated project" where 'project' is the
name of the project being updated.

Change it to "Updated git submodules" which is a bit more descriptive
of the actual change.

Change-Id: I4b54198c96bcbc7d6efdf7764704833751ecf5e5