log: load decorations with --simplify-by-decoration

log: load decorations with --simplify-by-decoration

It's possible to specify --simplify-by-decoration but not --decorate. In
this case we do respect the simplification, but we don't actually show
any decorations. However, it works by lazy-loading the decorations when
needed; this is discussed in more detail in 0cc7380d88 (log-tree: call
load_ref_decorations() in get_name_decoration(), 2019-09-08).

This works for basic cases, but will fail to respect any --decorate-refs
option (or its variants). Those are handled only when cmd_log_init()
loads the ref decorations up front, which is only when --decorate is
specified explicitly (or as of the previous commit, when the userformat
asks for %d or similar).

We can solve this by making sure to load the decorations if we're going
to simplify using them but they're not otherwise going to be displayed.

The new test shows a simple case that fails without this patch. Note
that we expect two commits in the output: the one we asked for by
--decorate-refs, and the initial commit. The latter is just a quirk of
how --simplify-by-decoration works. Arguably it may be a bug, but it's
unrelated to this patch (which is just about the loading of the
decorations; you get the same behavior before this patch with an
explicit --decorate).

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