revwalk: make mark_unintersting use a loop

Ask the odbbackend if the object exists before overwriting it

Now that #1785 is merged, git_odb_stream_finalize_write() calculates the object id before invoking the odb backend.

This commit gives a chance to the backend to check if it already knows this object.

Using a recursive function can blow the stack when dealing with long
histories. Use a loop instead to limit the call chain depth.

This fixes #1223.

Fix umask fragility

The GIT_MODE_TYPE macro was looking at all bits above the
permissions, but it should really just look at the top bits so
that it will give the right results for a setgid or setuid entry.

Since we're now using these macros in the tests, this was causing
a test failure on platforms that don't support setgid.

This fixes an issue checking file modes in the tests that
initialize a repo from a template directory when a symlink is
used in the template.  Also, this updates some other places where
we are examining file modes to use the new macros.

This adds some more macros for some standard operations on file
modes, particularly related to permissions, and then updates a
number of places around the code base to use the new macros.

Fix warning in src/win32/version.h

Some windows tests were failing

It seems that libgit2 is correctly applying the umask when
initializing a repository from a template and when creating new
directories during checkout, but the test suite is not accounting
for possible variations due to the umask.  This updates that so
that the test suite will work regardless of the umask.

Of backends and refreshers...

Previously, `git_object_read()`, `git_object_read_prefix()` and
`git_object_exists()` were implementing an auto refresh logic. When the
expected object couldn't be found in any backend, a call to
`git_odb_refresh()` was triggered and the lookup was once again performed
against all backends.

This commit removes this auto-refresh logic from the odb layer and pushes
it down into the pack-backend (as it's the only one currently exposing
a `refresh()` endpoint).

Split examples CMakeLists.txt

Also, this converts the examples/CMakeLists.txt from explicitly
listing to just globbing for all the individual C files.

Fix resolving relative windows network paths

Minor changes for rewrites

path: properly resolve relative paths

Fix incorrect precedence within git_repository_is_empty()

Small changes enabling compiling libgit2 for Android.

Small cleanup in examples.