"git checkout -p A...B [-- <path>]" did not work, even though the
same command without "-p" correctly used the merge-base between
commits A and B.
* dl/checkout-p-merge-base:
t2016: add a NEEDSWORK about the PERL prerequisite
add-patch: add NEEDSWORK about comparing commits
Doc: document "A...B" form for <tree-ish> in checkout and switch
builtin/checkout: fix `git checkout -p HEAD...` bug