Trust the detection of features and just test CoreConfig.

Change-Id: I37f838d270a221a75d0d3842ba2f3f90aa1d6f01

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>

Java has no option but to use precomposed Unicode, so we should
state that when creating a new repository. Not that Java will use
precomposed unicode regardless of this setting, but this reduces
the risk of incompatibility with C Git.

Change-Id: I3779b75f76d2e2061c836cbc9b4b7c2ae0cf18f4

Change-Id: I8f8ae7a8bc0793643184bc0e7afa9f4e2c5ff1a1
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

core.autorlf defaults to false, but can be set in the user or 
"system" config files. Note that EGit/JGit may not know
where the "system" config file is located.

Also fix pgm's ConfigTest which depends on default repository
configuration.

Bug: 382067
Change-Id: I2c698a76e30d968e7f351b4f5a2195f0b124f62f
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Currently, only --list option is supported with --global, --system,
--local and --file switches.

Change-Id: I9b179b162996520e95c4e001dccd65c566a4bd27
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

CQ: 6385
Bug: 365444
Change-Id: I2d5164cd92429673fe3c37e9f5f9bc565192cc12
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

jgit.storage.dht is a storage provider implementation for JGit that
permits storing the Git repository in a distributed hashtable, NoSQL
system, or other database.  The actual underlying storage system is
undefined, and can be plugged in by implementing 7 small interfaces:

  *  Database
  *  RepositoryIndexTable
  *  RepositoryTable
  *  RefTable
  *  ChunkTable
  *  ObjectIndexTable
  *  WriteBuffer

The storage provider interface tries to assume very little about the
underlying storage system, and requires only three key features:

  *  key -> value lookup (a hashtable is suitable)
  *  atomic updates on single rows
  *  asynchronous operations (Java's ExecutorService is easy to use)

Most NoSQL database products offer all 3 of these features in their
clients, and so does any decent network based cache system like the
open source memcache product.  Relying only on key equality for data
retrevial makes it simple for the storage engine to distribute across
multiple machines.  Traditional SQL systems could also be used with a
JDBC based spi implementation.

Before submitting this change I have implemented six storage systems
for the spi layer:

  * Apache HBase[1]
  * Apache Cassandra[2]
  * Google Bigtable[3]
  * an in-memory implementation for unit testing
  * a JDBC implementation for SQL
  * a generic cache provider that can ride on top of memcache

All six systems came in with an spi layer around 1000 lines of code to
implement the above 7 interfaces.  This is a huge reduction in size
compared to prior attempts to implement a new JGit storage layer.  As
this package shows, a complete JGit storage implementation is more
than 17,000 lines of fairly complex code.

A simple cache is provided in storage.dht.spi.cache.  Implementers can
use CacheDatabase to wrap any other type of Database and perform fast
reads against a network based cache service, such as the open source
memcached[4].  An implementation of CacheService must be provided to
glue this spi onto the network cache.

[1] https://github.com/spearce/jgit_hbase
[2] https://github.com/spearce/jgit_cassandra
[3] http://labs.google.com/papers/bigtable.html
[4] http://memcached.org/

Change-Id: I0aa4072781f5ccc019ca421c036adff2c40c4295
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Per CQ 3448 this is the initial contribution of the JGit project
to eclipse.org.  It is derived from the historical JGit repository
at commit 3a2dd9921c.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>