toolbox-version: grab only the commit hash instead of the commit message

This change prevent toolbox-version to keep too many lines when it updates
src/version.h, which was happening when the commit message contains the
word "commit".

Signed-off-by: Patrick Rouleau <pfrouleau@gmail.com>
Merge pull request #49 from pfrouleau/review/teach_toolbox-version_to_only_returns_the_commit_id

toolbox-version: grab only the commit hash instead of the commit message