revwalk: allow simplifying by first-parent

Make odb_loose return EAMBIGUOUS when required

When enabled, only the first parent of each commit will be queued,
enabling a simple way of using first-parent simplification.

fixing headers with bad values for objective-c

fixes issues with objective-git

odb: Error when streaming in less|more bytes than declared

Fix memory leaks

Some revparse love

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