Merge branch 'ss/submodule-summary-in-c'

Yet another subcommand of "git submodule" is getting rewritten in C.

* ss/submodule-summary-in-c:
  submodule: port submodule subcommand 'summary' from shell to C
  t7421: introduce a test script for verifying 'summary' output
  submodule: rename helper functions to avoid ambiguity
  submodule: remove extra line feeds between callback struct and macro

t/t7401-submodule-summary.sh

@@ -7,6 +7,12 @@ test_description='Summary support for submodules
 
 This test script tries to verify the sanity of summary subcommand of git submodule.
 '
+
+# NOTE: This test script uses 'git add' instead of 'git submodule add' to add
+# submodules to the superproject. Some submodule subcommands such as init and
+# deinit might not work as expected in this script. t7421 does not have this
+# caveat.
+#
 # NEEDSWORK: This test script is old fashioned and may need a big cleanup due to
 # various reasons, one of them being that there are lots of commands taking place
 # outside of 'test_expect_success' block, which is no longer in good-style.