Support git attrs from index (and bare repo)

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.

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

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

Cherry picked 2de60205dfea2c4a422b2108a5e8605f97c2e895 from
development into new-error-handling.

Conflicts:
	src/refspec.c

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