Support git attrs from index (and bare repo)

Honor core.notesRef config option

There are three changes here:
- correctly propogate error code from failed object lookups
- make zlib inflate use our allocators
- add OID to notfound error in ODB lookups

Depending on the operation, we need to consider gitattributes
in both the work dir and the index.  This adds a parameter to
all of the gitattributes related functions that allows user
control of attribute reading behavior (i.e. prefer workdir,
prefer index, only use index).

This fix also covers allowing us to check attributes (and
hence do diff and status) on bare repositories.

This was a somewhat larger change that I hoped because it had
to change the cache key used for gitattributes files.

Add git_note_default_ref to allow easy retrieval of the currently set
default notes reference.

Setting core.notesRef allows to change the default notes reference used
by Git. Check if set before using GIT_NOTES_DEFAULT_REF. Fixes #649.

Conflicts:
	.travis.yml
	include/git2/diff.h
	src/config_file.c
	src/diff.c
	src/diff_output.c
	src/mwindow.c
	src/path.c
	tests-clar/clar_helpers.c
	tests-clar/object/tree/frompath.c
	tests/t00-core.c
	tests/t03-objwrite.c
	tests/t08-tag.c
	tests/t10-refs.c
	tests/t12-repo.c
	tests/t18-status.c
	tests/test_helpers.c
	tests/test_main.c

Since strnlen is not supported on all platforms and since we
now have the shiny new git_text_is_binary in the filtering
code, let's convert diff binary detection to use the new stuff.

To make this code more resilient to future changes, we'll
explicitly translate the libgit2 structure to the libxdiff
structure.

This should restore the ability to include libgit2 headers
in C++ projects.

Cherry picked 2de60205 from
development into new-error-handling.

Conflicts:
	src/refspec.c

remote: don't free transport on disconnect

Currently, git_remote_disconnect not only closes the connection but also
frees the underlying transport object, making it impossible to write
code like

	// fetch stuff
	git_remote_download()

	// close connection
	git_remote_disconnect()

	// call user provided callback for each ref
	git_remote_update_tips(remote, callback)

because remote->refs points to references owned by the transport object.
This means, we have an idling connection while running the callback for
each reference.

Instead, allow immediate disconnect and free the transport later in
git_remote_free().

crlf normalization test coverage

Recognize and report server-side error messages

diff: provide more context to the consumer of the callbacks

When e.g. a repository isn't found, the server sends an error saying
so. Put that error message in our error buffer.

Update the callback to provide some information related to the file change being processed and the range of the hunk, when applicable.

Teach travis how to build the project.

The recent 64-bit Windows fixes changed the return code in
git_pkt_parse_line() so it wouldn't signal a short buffer, breaking
the network code. Bring it back.