Object validation tests for "jgit fsck"

Object validation tests for "jgit fsck"

The ObjectChecker provides strict validation rules for objects.  If
any commit, tag or tree object is malformed in a way that might cause
a Git implementation to misinterpret the data CorruptObjectException
is thrown back to the caller for error handling.

Due to the shear size of the validation code this change provides
only the validation code and its unit tests.  It is at least as
paranoid as git.git's "git-fsck" is on the same object types, but
is actually a bit stricter about the commit and tag objects having
the well known header fields populated correctly.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>