FS: Extract GobblerThread into a private static class

FS: Extract GobblerThread into a private static class

The primary goal is to improve exception readability. Since this is a
standalone thread, just logging the stack trace of the caught
exception is not very useful:

java.io.IOException: Stream closed
	at java.io.BufferedInputStream.getBufIfOpen(BufferedInputStream.java:162)
	at java.io.BufferedInputStream.read(BufferedInputStream.java:258)
	at org.eclipse.jgit.util.FS$2.run(FS.java:451)

Providing a named class eliminates the "FS$2", and including the
command name provides a little more context in the error message.
A future improvement might include the stack trace that created the
GobblerThread as well.

Change-Id: Ibf16d15b47a85b6f41844a177e398c2fc94f27b0