This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:

* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
  (i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
  the library and `giterr_last()` will return NULL if called.

This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.

This makes sure that an error code returned by the callback function
of `git_tree_walk` will stop the iteration and get propagated back
to the caller verbatim.

Also, this adds a minor helper function `git_tree_entry_byoid` that
searches a `git_tree` for an entry with the given OID.  This isn't
a fast function, but it's easier than writing the loop yourself as
an external user of the library.

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

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.