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.
Our previous assumption that all paths in Windows are encoded in UTF-8
is rather weak, specially when considering that Git is
encoding-agnostic.
These set of functions allow the user to change the library's active
codepage globally, so it is possible to access paths and files on all
international versions of Windows.
Note that the default encoding here is UTF-8 because we assume that 99%
of all Git repositories will be in UTF-8.
Also, if you use non-ascii characters in paths, anywhere, please burn on
a fire.
