branch-update-repo: add missing colon to catch the variable imgpath

Without this colon, the variable imgpath is not updated during branching.

See:

• tests/apertis-test-cases!672 (merged)
• tests/apertis-test-cases!671 (merged)

requested review from @wlozano

assigned to @daissi

mentioned in merge request tests/apertis-test-cases!672 (merged)

mentioned in merge request tests/apertis-test-cases!671 (merged)

mentioned in issue apertis-issues#735 (closed)

merged