Refactor our Maven build to be modular

Drop our simple and stupid jgit.sh and instead rely upon Maven
for the command line based build.  Maven is relatively simple to
download and install, and doesn't require the entire Eclipse IDE.

To avoid too much refactoring of the current code we reuse the
existing src/ directory within each plugin, and treat each of
the existing OSGI bundles as one Maven artifact.

The command line wrapper jgit.sh no longer works in the uncompiled
state, as we don't know where to obtain our JSch or args4j from.
Developers will now need to compile it with `mvn package`, or run
our Main class from within an IDE which has the proper classpath.

Bug: 291265
Change-Id: I355e95fa92fa7502651091d2b651be6917a26805
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>