Index tree-bob collision

If you are checking out NONE, then don't remove.

The checkout code used to defer removal of "blocking" files in
checkouts until the blocked item was actually being written (since
we have already checked that the removing the block is acceptable
according to the update rules).  Unfortunately, this resulted in
an intermediate index state where both the blocking and new items
were in the index which is no longer allowed.  Now we just remove
the blocking item in the first pass so it never needs to coexist.

In cases where there are typechanges, this could result in a bit
more churn of removing and recreating intermediate directories,
but I'm going to assume that is an unusual case and the churn will
not be too costly.

Fix errors with zstream (and a variety of other cleanups)

And don't terminate if there isn't space for it (since it's binary
data, it's not worth a reallocation).

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

When three-way merging indexes, we previously changed each path
as we read them, which would lead to us adding an index entry for
'foo', then removing an index entry for 'foo/file'.  With the new
index requirements, this is not allowed.  Removing entries in the
merged index, then adding them, resolves this.  In the previous
example, we now remove 'foo/file' before adding 'foo'.

In case insensitive index mode, we would stop at a prefixed entry,
treating the provided search key length as a substring, not the
length of the string to match.

Don't strcmp a git_buf, strcmp its char *