There were some confusing issues mixing up the number of bytes
written to the zstream output buffer with the number of bytes
consumed from the zstream input.  This reorganizes the zstream
API and makes it easier to deflate an arbitrarily large input
while still using a fixed size output.

This removes the fetchRecurse compiler warnings and makes the
behavior match the other submodule options (i.e. the in-memory
setting can be reset to the on-disk value).

We never search for libiconv via pkg-config

So we actually also never know that we can set a dependency on it in
pkg-config. Instead always give it the -L and -l options.

Fixing #2017 and #2087

- Add correct -I, -L and -l flags
- Search for libiconv in /opt/local/[include|lib] before in the
  system path. See #2017 for details.
- Give splitted -L and -l arguments to pkg-config

Doesn't change anything. Just removes stuff that was probably missed
to remove when this was imported.

This has actually no effect on a "normal" build, but allows to use
libgit2 as a part of a larger project via CMake's ADD_SUBDIRECTORY()

Closes #2087

Don't strcmp a git_buf, strcmp its char *

Add `git_commit_descendant_of`.

Add built in diff drivers

Writing a sample Javascript driver pointed out some extra
whitespace handling that needed to be done in the diff driver.
This adds some tests with some sample javascript code that I
pulled off of GitHub just to see what would happen.  Also, to
clean up the userdiff test data, I did a "git gc" and packed
up the test objects.

Leftover OID -> ID changes

Buff up returning strings

MSVC doesn't like modern code neither

Make blame example compile on MSVC

A lot of the tests were checking for overflow, which we don't have
anymore, so we can remove them.

Again, we already did this internally, so simply remove the conversions.

Internally we already did everything with git_bufs, so this is just
exposing those functions with public names.

Since we now export that type, we can avoid making the user guess a
size.

Fix git_remote_dup & add missing const

Fix write_object.

Fix local push to file:// URL.

Following the rest of the series, use 'id' when refering to the value.

Same as the other commits in the series, we use 'id' when talking about
thing rather than the datatype.