The git-show-ref(1) command has three different modes, of which one is
implicit and the other two can be chosen explicitly by passing a flag.
But while these modes are standalone and cause us to execute completely
separate code paths, we gladly accept the case where a user asks for
both `--exclude-existing` and `--verify` at the same time even though it
is not obvious what will happen. Spoiler: we ignore `--verify` and
execute the `--exclude-existing` mode.

Let's explicitly detect this invalid usage and die in case both modes
were requested.

Signed-off-by: Patrick Steinhardt <redacted>
---

diff --git a/builtin/show-ref.c b/builtin/show-ref.c

@@ -269,6 +269,9 @@ int cmd_show_ref(int argc, const char **argv, const char *prefix)
+       if ((!!exclude_existing_opts.enabled + !!verify) > 1)
+               die(_("only one of --exclude-existing or --verify can be given"));