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.

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

refs: fix a memory leak

    passing 0 to git_strol(32|64) let the implementation guess if it's
    dealing with an octal number or a decimal one.

    Let's make it safe and ensure that both 'HEAD@{010}' and 'HEAD@{10}'
    point at the same commit.

khash.h was globally #define'ing "inline" which messes with
other files.  Let's keep it as "kh_inline".

Some objects were added in another PR

odb: add git_odb_foreach()

Fix revwalk email parsing

Revparse refactoring: a start

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