use xstrdup_or_null to replace ternary conditionals

This replaces "x ? xstrdup(x) : NULL" with xstrdup_or_null(x).
The change is fairly mechanical, with the exception of
resolve_refdup, which can eliminate a temporary variable.

There are still a few hits grepping for "?.*xstrdup", but
these are of slightly different forms and cannot be
converted (e.g., "x ? xstrdup(x->foo) : NULL").

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/remote.c b/remote.c
index ce785f8..179bcef 100644
--- a/remote.c
+++ b/remote.c
@@ -975,8 +975,8 @@
 	cpy = xmalloc(sizeof(struct ref) + len + 1);
 	memcpy(cpy, ref, sizeof(struct ref) + len + 1);
 	cpy->next = NULL;
-	cpy->symref = ref->symref ? xstrdup(ref->symref) : NULL;
-	cpy->remote_status = ref->remote_status ? xstrdup(ref->remote_status) : NULL;
+	cpy->symref = xstrdup_or_null(ref->symref);
+	cpy->remote_status = xstrdup_or_null(ref->remote_status);
 	cpy->peer_ref = copy_ref(ref->peer_ref);
 	return cpy;
 }