Add support for built-in clean filters

Add support for built-in clean filters

JGit supports clean filters defined in repository configuration. The
filters are implemented as external programs filtering content by
accepting the original content (as seen in the working tree) on stdin
and which emit the filtered content on stdout. To run such a filter JGit
has to start an external process and pump data into/from this process.

This commit adds support for clean filters which are implemented
in Java and which are executed by jgit's main thread. When a filter is
defined in the configuration as
"jgit://builtin/<filterDriverName>/clean" then JGit will lookup in a
static map whether a filter is registered under this name. If found
such a filter is called to do the filtering.

The functionality in this commit requires that a program using JGit
explicitly calls the JGit API to register built-in implementations for
specific clean filters. In follow-up commits configuration parameters
will be added which trigger such registrations. Other commits will add
implementations for lfs filters.

Change-Id: I0344d3c54801c9a46e5a606c5df17e5f2e17b2be