ObjectChecker: Report .gitmodules files found in the pack

ObjectChecker: Report .gitmodules files found in the pack

In order to validate .gitmodules files, we first need to find them
in the incoming pack.

Do it in the ObjectChecker stage. Check in the tree objects if they
point to a .gitmodules file and report the tree id and the .gitmodules
blob id.

This can be used later to check if the file is in the root of the
project and if the contents are good.

While we're here, make isMacHFSGit more accurate by detecting variants
of filenames that vary in case.

[jn: tweaked NTFS and HFS+ checking; added more tests]

Change-Id: I70802e7d2c1374116149de4f89836b9498f39582
Signed-off-by: Ivan Frade <ifrade@google.com>
Signed-off-by: Jonathan Nieder <jrn@google.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>