Unlike ignore files, gitattribute files can have flexible whitespace at
the beginning of the line.  Ensure that by adding new ignore rules that
we have not impeded correct parsing of attribute files.

Removed the BUILD_CLAR CMake flag, and updated the readme.

Once I added tests for the whitespace handling options of
diff, I realized that there were some bugs.  This fixes
those and adds the new tests into the test suite.

This gets the basic plumbing in place for git_diff_blob.
There is a known issue where additional parameters like
the number of lines of context to display on the diff
are not working correctly (which leads one of the new
unit tests to fail).

This is a test that should replicate an issue that Peff
is setting with git attributes.  But the test doesn't fail.

Add unit tests to confirm ignore directory pattern matches and
to confirm that ignore and attribute files are loaded properly
into the attribute file cache.

Per issue #533, the handling of relative paths in attribute
and ignore files was not right.  Fixed this by pre-joining
the relative path of the attribute/ignore file onto the match
string when a full path match is required.

Unfortunately, fixing this required a bit more code than I
would have liked because I had to juggle things around so that
the fnmatch parser would have sufficient information to prepend
the relative path when it was needed.

This fixes issue 532 that attributes (and gitignores) could not
be checked for files that don't exist.  It should be possible to
query such things regardless of the existence of the file.

This updates to implementation of gitattribute macros to be much more
similar to core git (albeit not 100%) and to handle expansion of
macros within macros, etc.  It also cleans up the refcounting usage
with macros to be much cleaner.

Also, this adds a new vector function `git_vector_insert_sorted()`
which allows you to maintain a sorted list as you go.  In order to
write that function, this changes the function `git__bsearch()` to
take a somewhat different set of parameters, although the core
functionality is still the same.