scripts/oe-setup-layers: Update how to determine if directory is git repo

scripts/oe-setup-layers: Update how to determine if directory is git repo

Previously _is_repo_git_repo() would return a result containing b'true\n' or
b'false\n' if 'git rev-parse' ran successfully, instead of True of False.

While this can be solved using e.g. result.strip().decode("utf-8") == "true",
there are some other cases to consider.
First, .git can be a file and not a directory when using a worktree.
Second, an emtpy .git directory in 'repodir' for which some parent of
'repodir' is an actual git repo will still return True in this case.

To account for these cases as well, use 'git rev-parse --show-toplevel'
and compare the result against 'repodir' instead of
using 'git rev-parse --is-inside-git-dir'.

Signed-off-by: Jermain Horsman <jermain.horsman@nedap.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>