git_tests: Use Brach.full_name instead of '{}/{}'.format().
git: Add a new method to Branch: unmerged_commits().
git: Run 'git show' with '--quiet'.
web: Show unmerged commits in each branch.
git_tests: Create a new base class for all unmerged-commits-related tests.
git: Add a new method to Repo: has_unmerged_commits().
git: Add a new method to Branch: has_unmerged_commits().
web: Use Branch.has_unmerged_commits() to find whether a branch has unmerged commits.
git: Add a new parameter to get_unmerged_commits(): limit.
web: Limit the number of unmerged commits that are shown.