checkout.c: use ref_exists instead of file_exist

Change checkout.c to check if a ref exists instead of checking if a loose ref
file exists when deciding if to delete an orphaned log file. Otherwise, if a
ref only exists as a packed ref without a corresponding loose ref for the
currently checked out branch, we risk that the reflog will be deleted when we
switch to a different branch.

Update the reflog tests to check for this bug.

The following reproduces the bug:
$ git init-db
$ git config core.logallrefupdates true
$ git commit -m Initial --allow-empty
    [master (root-commit) bb11abe] Initial
$ git reflog master
    [8561dcb master@{0}: commit (initial): Initial]
$ find .git/{refs,logs} -type f | grep master
    [.git/refs/heads/master]
    [.git/logs/refs/heads/master]
$ git branch foo
$ git pack-refs --all
$ find .git/{refs,logs} -type f | grep master
    [.git/logs/refs/heads/master]
$ git checkout foo
$ find .git/{refs,logs} -type f | grep master
    ... reflog file is missing ...
$ git reflog master
    ... nothing ...

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Acked-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1410-reflog.sh

@@ -245,4 +245,12 @@ test_expect_success 'gc.reflogexpire=false' '
 
 '
 
+test_expect_success 'checkout should not delete log for packed ref' '
+	test $(git reflog master | wc -l) = 4 &&
+	git branch foo &&
+	git pack-refs --all &&
+	git checkout foo &&
+	test $(git reflog master | wc -l) = 4
+'
+
 test_done