Also, now only reporting checkout progress for files that
are actually being added or removed.

The fetch code takes advantage of this to implement a
progress callback every 100kb of transfer.

Also implemented in the git2 example.

git_index_read_tree() was exposing a parameter to provide the user with
a progress indicator. Unfortunately, due to the recursive nature of the
tree walk, the maximum number of items to process was unknown. Thus,
the indicator was only counting processed entries, without providing
any information how the number of remaining items.

Also converted the network example to use it.

Also removing all the *stats parameters from external
APIs that don't need them anymore.

Changes to CMake for packaging

Fix env variable tests with new Win32 path rules

Rename INSTALL_INC and INSTALL_BIN to INCLUDE_INSTALL_DIR and
BIN_INSTALL_DIR, which are more commonly used. This is also
consistent with the variable for the library path which is
already LIB_INSTALL_DIR.

The new Win32 global path search was not working with the
environment variable tests.  But when I fixed the test, the new
codes use of getenv() was causing more failures (presumably because
of caching on Windows ???).  This fixes the global file lookup to
always go directly to the Win32 API in a predictable way.

Add error GIT_EORPHANEDHEAD

Update README because vmg is @vmg.

Enhance test coverage