Fix "checkout A..." synonym for "checkout A...HEAD" on Windows

Fix "checkout A..." synonym for "checkout A...HEAD" on Windows

When switching to a different commit, we first see the named rev exists
as a commit using lookup_commit_reference_gently(), and set new.path to
a string "refs/heads/" followed by the name the user gave us (but after
taking into special short-hands like @{-1} == "previous branch" and
"@{upstream}" == "the branch we merge with" into account).  If the
resulting string names an existsing ref, then we are switching to that
branch (and will be building new commits on top of it); otherwise we are
detaching HEAD at that commit.

When the "master..." syntax is used as a short-hand for "master...HEAD",
we do want to detach HEAD at the merge base.  However, on Windows, when
asked if ".git/refs/heads/master..." exists, the filesystem happily says
"it does" when ".git/refs/heads/master" exists.

Work this issue around by first calling check_ref_format(new.path) to see
if the string can possibly be a valid ref under "refs/heads/", before
asking resolve_ref().

We used to run another lookup_commit_reference(rev) even though we know it
succeeded and we have a good commit in new.commit already; this has been
with us from 782c2d6 (Build in checkout, 2008-02-07), the first version we
had "git checkout" implemented in C.  Drop it.

Noticed by Alex Riesen.

Signed-off-by: Junio C Hamano <gitster@pobox.com>