BUG: 2.11-era rebase regression when @{upstream} is implicitly used

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

This is not a 2.21 release issue, and pre-dates the built-in rebase.

When you clone any repository, e.g. git.git, and add one commit on top
of the cloned branch, then run "git rebase" you'll get e.g.:

    $ git rebase
    First, rewinding head to replay your work on top of it...
    Applying: foo

Before 4f21454b55 ("merge-base: handle --fork-point without reflog",
2016-10-12) you'd get:

    $ git rebase
    Current branch master is up to date.

The results are not the same for "git rebase @{u}" or "git rebase $(git
rev-parse @{u})":

    $ git rev-parse HEAD; ~/g/git/git --exec-path=/home/avar/g/git rebase; git rev-parse HEAD; ~/g/git/git --exec-path=/home/avar/g/git rebase @{u}; git rev-parse HEAD; ~/g/git/git --exec-path=/home/avar/g/git rebase $(git rev-parse @{u}); git rev-parse HEAD
    d0a1e49341cac6db3226eb0f76ec4a5912f18af8
    First, rewinding head to replay your work on top of it...
    Applying: foo
    3a9261d6e34d9f6d00c8e8411d7ddd8cffa02d97
    Current branch master is up to date.
    3a9261d6e34d9f6d00c8e8411d7ddd8cffa02d97
    Current branch master is up to date.
    3a9261d6e34d9f6d00c8e8411d7ddd8cffa02d97

With 4f21454b55^ checked-out the SHA-1 always stays the same, i.e. no
work is done for the same command:

    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b
    Current branch master is up to date.
    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b
    Current branch master is up to date.
    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b
    Current branch master is up to date.
    69bd93d9aa438c6d903b8e62c3bf1c6c5ab8ec0b