rebase: teach rebase --keep-base

A common scenario is if a user is working on a topic branch and they
wish to make some changes to intermediate commits or autosquash, they
would run something such as

	git rebase -i --onto master... master

in order to preserve the merge base. This is useful when contributing a
patch series to the Git mailing list, one often starts on top of the
current 'master'. While developing the patches, 'master' is also
developed further and it is sometimes not the best idea to keep rebasing
on top of 'master', but to keep the base commit as-is.

In addition to this, a user wishing to test individual commits in a
topic branch without changing anything may run

	git rebase -x ./test.sh master... master

Since rebasing onto the merge base of the branch and the upstream is
such a common case, introduce the --keep-base option as a shortcut.

This allows us to rewrite the above as

	git rebase -i --keep-base master

and

	git rebase -x ./test.sh --keep-base master

respectively.

Add tests to ensure --keep-base works correctly in the normal case and
fails when there are multiple merge bases, both in regular and
interactive mode. Also, test to make sure conflicting options cause
rebase to fail. While we're adding test cases, add a missing
set_fake_editor call to 'rebase -i --onto master...side'.

While we're documenting the --keep-base option, change an instance of
"merge-base" to "merge base", which is the consistent spelling.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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