Add support for push-urls

Fix/issue 787

Asking the library for "" used to give HEAD, but that's trying to
impose a default at the wrong layer. Make it fail.

Revparse rework

This allows us to set options like "some.foo\\ish.var".

This closes #830

tree: fix ordering for git_tree_walk

A diff that is created with a NULL options parameter could result
in a NULL prefix string, but diff merge was unconditionally
strdup'ing it.  I added a test to replicate the issue and then a
new method that does the right thing with NULL values.

Josh Triplett noticed libgit2 actually does preorder entries in
tree_walk_post instead of postorder. Also, we continued walking even
when an error occured in the callback.

Fix #773; also, allow both pre- and postorder walking.

Partially fix #530

Remove old error handling code

libgit2.pc.in: also switch to LIB_INSTALL_DIR