Add GIT_DIFF_FIND_REMOVE_UNMODIFIED flag and fix copy detection bug

When doing copy detection, it is often necessary to include
UNMODIFIED records in the git_diff so they are available as source
records for GIT_DIFF_FIND_COPIES_FROM_UNMODIFIED.  Yet in the final
diff, often you will not want to have these UNMODIFIED records.
This adds a flag which marks these UNMODIFIED records for deletion
from the diff list so they will be removed after the rename detect
phase is over.

When FIND_COPIES is used in combination with BREAK_REWRITES for
rename detection, there was a bug where the split MODIFIED delta
was only used as a target for RENAME records and not for COPIED
records.  This fixes that, converting the split into a pair of
DELETED and COPIED deltas when that circumstance arises.

Improve iconv finding for cmake

* add FindIconv helper for CMake iconv detection
* only default using iconv to ON for MacOS
* update pkg-config generation to include iconv dependency better

fix typos in docs

posix: Solaris doesn't have strnlen either

ssh: add support for ssh-agent authentication

Apologies for the delay. This release is chunky,
but you're probably used to chunky releases by now.

Full changelog follows:

Internal changes
	+ Added support for decomposed Unicode paths in Mac OS X
	+ Added support for junctions in win32
	+ Fixed issues with HTTP redirects in the network stack
	+ Performance improvements (as always)

git2/blame.h
	+ added full support for blame operations

git2/blob.h
	+ added `git_blob_filtered_content` to load blobs in
	memory with their corresponding filters applied

git2/branch.h
	+ added branch iterators as an alternative to the
	callback-based API

git2/buffer.h
	+ exported the git_buf struct to allow binary buffers
	to be passed from/to the library. This simplifies
	several APIs

git2/checkout.h
	+ implemented additional checkout options ('skip unmerged',
	'use ours' and 'use theirs')

git2/clone.h
	+ simplified the clone API
	+ added new `git_clone_into` to clone into an existing
	(empty) repository

git2/commit.h
	+ added APIs to access the raw (uncleaned) text of a commit
	message

git2/common.h
	+ added global options to set the default paths to load
	templates from

git2/config.h
	+ added multivar iterators
	+ added globbing iterator
	+ added `git_config_foreach_match` to perform operations
	on every single var in a config file

git2/diff.h
	+ restructured and simplified the diff API

git2/filter.h
	+ added external APIs to configure and apply custom filters
	to checked out blobs

git2/index.h
	+ `git_index_read` can now force a reload of the index file
	even if it hasn't changed on-disk

git2/indexer.h
	+ improved the streaming indexer APIs

git2/merge.h
	+ added support for merge!

git2/object.h
	+ added helper API to lookup a generic object by path

git2/pack.h
	+ added progress callbacks to the packbuilder

git2/patch.h
	+ added support for patch generation as part of the Diff
	API revamp

git2/pathspec.h
	+ added helper APIs to work with pathspecs and match files
	in the workdir or diffs

git2/push.h
	+ added progress callbacks to push

git2/reflog.h
	+ changed reflog APIs to work on reference names instead of
	the references themselves, so they become less racy

git2/remote.h
	+ added support for setting refspecs on remotes, either at
	creation or on existing ones
	+ simplified the remotes API

git2/revwalk.h
	+ add API to simplify parents during a walk

git2/signature.h
	+ add helper to create a signature with the default values for
	a repository (i.e. the set `user.name` and `user.email` values,
	and the current time)

git2/submodules.h
	+ improve the status detection for submodules

git2/sys/
	+ exported many new internal APIs, such as pluggabe transport
	APIs

-vmg, out

Add content offset to git_diff_line

util: NetBSD doesn't have qsort_r either

Cleaning up

Corrupted index is bad for your health

Cleaning code, removing unused variables

For additions and deletions, external consumers like subversion
can make use of the content offset to generate diffs in their
proprietary formats.

NTLM/Negotiate support in WinHTTP

Add missing mappings to .mailmap file

Rename tests directory

Allowing create remotes with custom fetch spec