builtin/mv: fix leaks for submodule gitfile paths

Similar to the preceding commit, we have effectively given tracking
memory ownership of submodule gitfile paths. Refactor the code to start
tracking allocated strings in a separate `struct strvec` such that we
can easily plug those leaks. Mark now-passing tests as leak free.

Note that ideally, we wouldn't require two separate data structures to
track those paths. But we do need to store `NULL` pointers for the
gitfile paths such that we can indicate that its corresponding entries
in the other arrays do not have such a path at all. And given that
`struct strvec`s cannot store `NULL` pointers we cannot use them to
store this information.

There is another small gotcha that is easy to miss: you may be wondering
why we don't want to store `SUBMODULE_WITH_GITDIR` in the strvec. This
is because this is a mere sentinel value and not actually a string at
all.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7421-submodule-summary-add.sh

@@ -10,6 +10,7 @@ while making sure to add submodules using `git submodule add` instead of
 `git add` as done in t7401.
 '
 
+TEST_PASSES_SANITIZE_LEAK=true
 . ./test-lib.sh
 
 test_expect_success 'setup' '