Add symlink support to JGit

Add symlink support to JGit

The change includes comparing symbolic links between disk and index,
adding symbolic links to the index, creating/modifying links on
checkout. The behavior is controlled by the core.symlinks setting, just
as C Git does. When a new repository is created core.symlinks will be
set depending on the capabilities of the operating system and Java
runtime.

If core.symlinks is set to true, the assumption is that symlinks are
supported, which may result in runtime errors if this turns out not to
be the case.

Measuring the cost of jgit status on a repository with ~70000 files,
of which ~30000 are tracked reveals a penalty of about 10% for using
the Java7 (really NIO2) support module.

Bug: 354367
Change-Id: I12f0fdd9d26212324a586896ef7eb1f6ff89c39c
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>