Removed the BUILD_CLAR CMake flag, and updated the readme.

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

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.

Add support for git attribute macro definitions.  Also, add
support for cache flush API to clear the attribute file content
cache when needed.

Additionally, improved the handling of global and system files,
making common utility functions in fileops and converting config
and attr to both use the common functions.

Adds a bunch more tests and fixed some memory leaks.  Note that
adding macros required me to use refcounted attribute assignment
definitions, which complicated, but probably improved memory usage.