Add content offset to git_diff_line

For additions and deletions, external consumers like subversion
can make use of the content offset to generate diffs in their
proprietary formats.
Merge pull request #1966 from nickh/patch_content_offsets

Add content offset to git_diff_line
Formatting fix for cred_acquire_cb
libgit2 0.20.0 "anmeldung"

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
ssh: add support for ssh-agent authentication
transport: document ssh-agent authentication