quoted
git merge-base --all <branch1> <branch2>
Be warned: there might be multiple merge bases.
I do not think that approach applies to linux-next, which is constantly
rewound to the then-tip-of-linus and merge remaining bits. The question
is "where does this branch begin", which does not have an answer in git.
Using git merge-base on the next-20080701 tag and current Linus tree I get
76 possible merge bases. None of them appear to be the "right" one (if
I check out this tag and look at Next/merge.log the right answer appears
to be 1702b52 if I'm reading the log correctly).
Perhaps my best hope is to
$ git checkout $tag Next/merge.log
... parse merge.log to figure out $base ...
-Tony