Fix second manpage test

Using `git diff-index` is problematic for this test, because manpages
regenerated by `make` will have a modified mtime, and `git diff-index`
(being a low-level command) does not refresh the file's mtime stored in
the index. Refreshing the mtime and other stat information stored in the
index is something that happens automatically for high-level "porcelain"
commands.

Since `git diff` will suffice for checking that no uncommitted changes
to manpages exist, use it instead.

Also, use the --exit-code option instead to see the diff output in case
of failure.

Signed-off-by: Patrick McCarty <patrick.mccarty@intel.com>

test/documentation/manpages/test.bats

@@ -22,7 +22,7 @@ setup() {
   do
     [ -f "$nroff" ] || continue
     echo "$nroff"
-    git diff-index --quiet HEAD "$nroff"
+    git diff --exit-code HEAD "$nroff"
   done
 }