git-rebase-update - ensure all branches are up to date
git-new-branch - create branches
git-rename-branch - rename a branch while preserving parentage relationships
git-reparent-branch - change the parent of a branch, including rebasing it correctly onto that new parent.
git-squash-branch - collapse a branch into a single commit
git-upstream-diff - show the diff between the current branch and it's upstream branch
git-mark-merge-base - explicitly set what you want the above tools to consider the merge-base for the current branch.

R=agable@chromium.org, hinoka@chromium.org, stip@chromium.org, szager@chromium.org
BUG=261738

Review URL: https://codereview.chromium.org/184253003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@259520 0039d316-1c4b-4281-b951-d872f2087c98