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.

Move `git_ref_t` to `git_reference_t`

Update some missed types that were continuing to use the old `GIT_OBJ`
names.

Update internal usage to use the `git_reference` names for constants.

I don't get how it was working without git_libgit2_init() call. I run it
and libgit2 throws assertion somewhere in its internals. Now it works.

Updated commit with shutdown at the end.

This is quite close to running "git for-each-ref" except:

  1. It does not take any formatting or selection options at
     all.

  2. The output is not sorted.

I wrote it to look at debugging some issues with ref
iteration, but there's no reason it can't live on as an
example command.