--metadata-file: when multiple files specified, second takes precedence...

on conflicting fields.  This changes earlier behavior (but not in
a release), where first took precedence.

Note that this may seem inconsistent with the behavior of
multiple YAML blocks within a document, where the first takes
precedence.  Still, it is convenient to be able to override
defaults with options later on the command line.

test/command/5700-metadata-file-1.yml

@@ -1 +1 @@
-title: Multiple metadata files test
+title: This title should be overridden by 5700-metadta-file-2.yml

test/command/5700-metadata-file-2.yml

@@ -1,2 +1,2 @@
-title: This title should be overridden by 5700-metadta-file-2.yml
+title: Multiple metadata files test
 desc: Both of these files should be loaded.