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

b3237ac3 changed CMakeList.txt to use the more standard LIB_INSTALL_DIR
variable instead of INSTALL_LIB, but forgot to change libgit2.pc.in etc.
accordingly.

Add support for the more standard LIB_INSTALL_DIR variable

Pevents collisions with the original libgit, which also exports those
exact symbols.

Fixes #822

Not all delta bases are available on the first try. By delaying
resolving all deltas until the end, we avoid decompressing some of the
data twice or even more times, saving effort and time.

The correct way to advertise out capabilities is by appending them to
the first 'want' line, using SP as separator, instead of NUL as the
server does. Inconsistent documentation lead to the use of NUL in
libgit2.

Fix this so we can request much more efficient packs from the
remote which reduces the indexing time considerably.

More revparse <3