Skip logging stack trace on corrupt objects

Skip logging stack trace on corrupt objects

Instead of dumping a full stack trace when a client sends an invalid
commit, record only a short line explaining the attempt:

  Cannot receive Invalid commit c0ff33...: invalid author into /tmp/jgit.git

The text alone is sufficient to explain the problem and the stack
trace does not lend any additional useful information. ObjectChecker
is quite clear about its rejection cases.

Change-Id: Ifc8cf06032489dc6431be1ba66101cf3d4299218