Re: Find the starting point of a local branch

From: Kevin <ikke@ikke.info>
Date: Dec 24, 2012, 09:12 UTC
Message-ID: <CAO54GHDPaBqLGV7nNS-fwMZV4t_XCfDA-tAx5w7SrWWsM+YBGA@mail.gmail.com>
In-Reply-To: <20121224073103.GA10793@zuhnb712>

On Mon, Dec 24, 2012 at 8:31 AM, Woody Wu <narkewoody@gmail.com> wrote:

> But thanks anyway, I see you guys's discussions and it's a little hard
> to understand to me at the moment. Currently, I still have to use gitk
> with narrowed outputs.

Each commit refers to it's parent. If you take a branch, and keep following the first parent of the commits, you will finally reach the root commit (the first commit made). You don't see any branches, it's just one straight line. That's why git can't tell you where a branch started, because it all looks like a single branch to git.

With merge-base, you basically give git two branches, and git finds the first ancestor that is common to both branches.

See [1] for an example. If you follow the arrows from branch a, you get from E to A (note that commit C doesn't know about commit F pointing to it). So it looks like there is only a single branch.

If you do git merge-base a b, git sees both branches have commit C in common, and reports that as the merge-base.

[1]: http://g.jk.gs/9W.png