submodule absorbgitdirs tests: add missing "Migrating git..." tests

submodule absorbgitdirs tests: add missing "Migrating git..." tests

Fix a blind spots in the tests surrounding "submodule absorbgitdirs"
and test what output we emit, and how emitted the message and behavior
interacts with a "git worktree" where the repository isn't at the base
of the working directory.

The "$(pwd)" instead of "$PWD" here is needed due to Windows, where
the latter will be a path like "/d/a/git/[...]", whereas we need
"D:/a/git/[...]".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>