am: fix --interactive HEAD tree resolution

am: fix --interactive HEAD tree resolution

In --interactive mode, "git am --resolved" will try to generate a patch
based on what is in the index, so that it can prompt "apply this
patch?". To do so it needs the tree of HEAD, which it tries to get with
get_oid_tree(). However, this doesn't yield a tree object; the "tree"
part just means "if you must disambiguate short oids, then prefer trees"
(and we do not need to disambiguate at all, since we are feeding a ref).

Instead, we must parse the oid as a commit (which should always be true
in a non-corrupt repository), and access its tree pointer manually.

This has been broken since the conversion to C in 7ff2683253
(builtin-am: implement -i/--interactive, 2015-08-04), but there was no
test coverage because of interactive-mode's insistence on having a tty.
That was lifted in the previous commit, so we can now add a test for
this case.

Note that before this patch, the test would result in a BUG() which
comes from 3506dc9445 (has_uncommitted_changes(): fall back to empty
tree, 2018-07-11). But before that, we'd have simply segfaulted (and in
fact this is the exact type of case the BUG() added there was trying to
catch!).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>