This restores the ability to build using just Eclipse without
strange procedures, extra plugins and it is again possible to
work on both JGit and EGit in the same Eclipse workspace with
ease.

Change-Id: I0af08127d507fbce186f428f1cdeff280f0ddcda
Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>

Tycho isn't production ready for projects like JGit to be using as
their primary build driver.  Some problems we ran into with Tycho
0.6.0 that are preventing us from using it are:

 * Tycho can't run offline

   The P2 artifact resolver cannot perform its work offline.  If the
   build system has no network connection, it cannot compile a
   project through Tycho.  This is insane for a distributed version
   control system where developers are used to being offline during
   development and local testing.

 * Magic state in ~/.m2/repository/.meta/p2-metadata.properties

   Earlier iterations of this patch tried to use a hybrid build,
   where Tycho was only used for the Eclipse specific feature and P2
   update site, and maven-bundle-plugin was used for the other code.
   This build seemed to work, but only due to magic Tycho specific
   state held in my local home directory.  This means builds are not
   consistently repeatable across systems, and lead me to believe
   I had a valid build, when in fact I did not.

 * Manifest-first build produces incomplete POMs

   The POM created by the manifest-first build format does not
   contain the dependency chain, leading a downstream consumer to
   not import the runtime dependencies necessary to execute the
   bundle it has imported.  In JGit's case, this means JSch isn't
   included in our dependency chain.

 * Manifest-first build produces POMs unreadable by Maven 2.x

   JGit has existing application consumers who are relying on
   Maven 2.x builds.  Forcing them to step up to an alpha release
   of Maven 3 is simply unacceptable.

 * OSGi bundle export data management is tedious

   Editing each of our pom.xml files to mark a new release is
   difficult enough as it is.  Editing every MANIFEST.MF file to
   list our exported packages and their current version number is
   something a machine should do, not a human.  Yet the Tycho OSGi
   way unfortunately demands that a human do this work.

 * OSGi bundle import data management is tedious

   There isn't a way in the MANIFEST.MF file format to reuse the
   same version tags across all of our imports, but we want to have
   a consistent view of our dependencies when we compile JGit.

After wasting more than 2 full days trying to get Tycho to work,
I've decided its a lost cause right now.  We need to be chasing down
bugs and critical features, not trying to bridge the gap between
the stable Maven repository format and the undocumented P2 format
used only by Eclipse.

So, switch the build to use Apache Felix's maven-bundle-plugin.

This is the same plugin Jetty uses to produce their OSGi bundle
manifests, and is the same plugin used by the Apache Felix project,
which is an open-source OSGi runtime.  It has a reasonable number
of folks using it for production builds, and is running on top of
the stable Maven 2.x code base.

With this switch we get automatically generated MANIFEST.MF files
based on reasonably sane default rules, which reduces the amount
of things we have to maintain by hand.  When necessary, we can add
a few lines of XML to our POMs to tweak the output.

Our build artifacts are still fully compatible with Maven 2.x, so
any downstream consumers are still able to use our build products,
without stepping up to Maven 3.x.  Our artifacts are also valid as
OSGi bundles, provided they are organized on disk into a repository
that the runtime can read.

With maven-bundle-plugin the build runs offline, as much as Maven
2.x is able to run offline anyway, so we're able to return to a
distributed development environment again.

By generating MANIFEST.MF at the top level of each project (and
therefore outside of the target directory), we're still compatible
with Eclipse's PDE tooling.  Our projects can be imported as standard
Maven projects using the m2eclipse plugin, but the PDE will think
they are vaild plugins and make them available for plugin builds,
or while debugging another workbench.

This change also completely removes Tycho from the build.

Unfortunately, Tycho 0.6.0's pom-first dependency resolver is broken
when resolving a pom-first plugin bundle through a manifest-first
feature package, so bundle org.eclipse.jgit can't be resolved,
even though it might actually exist in the local Maven repository.

Rather than fight with Tycho any further, I'm just declaring it
plugina-non-grata and ripping it out of the build.

Since there are very few tools to build a P2 format repository, and
no documentation on how to create one without running the Eclipse
UI manually by poking buttons, I'm declaring that we are not going
to produce a P2 update site from our automated builds.

Change-Id: If7938a86fb0cc8e25099028d832dbd38110b9124
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>