Consolidate all standard includes and defines into "common.h". This lets
us avoid having to handle platform-specific things in multiple places.

Inside of our networking example code, we have a git2 executable
that acts as an entry point to all the different network
examples. As such, it is kind of the same like the normal git(1)
executable in that it simply arbitrates to the respective
subcommands.

Let's extend this approach and merge all examples into a single
standalone lg2 executable. Instead of building an executable
for all the existing examples we have, we now bundle them all
inside of the lg2 one and let them be callable via subcommands.

In the process, we can get rid of duplicated library
initialization, deinitialization and repository discovery code.
Instead of having each subcommand handle these on its own, we
simply do it inside of the single main function now.

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.

This allows you to use a --repeat option to run status over and
over and see how the output changes as you make local directory
changes without reopening the git_repository object each time.

Also, adds a flag to explicitly list the submodules before status.

`git_submodule` objects were already refcounted internally in case
the submodule name was different from the path at which it was
stored.  This makes that refcounting externally used as well, so
`git_submodule_lookup` and `git_submodule_add_setup` return an
object that requires a `git_submodule_free` when done.