Previously the git_diff_delta recorded if the delta was binary.
This replaces that (with no net change in structure size) with
a full set of flags.  The flag values that were already in use
for individual git_diff_file objects are reused for the delta
flags, too (along with renaming those flags to make it clear that
they are used more generally).

This (a) makes things somewhat more consistent (because I was
using a -1 value in the "boolean" binary field to indicate unset,
whereas now I can just use the flags that are easier to understand),
and (b) will make it easier for me to add some additional flags to
the delta object in the future, such as marking the results of a
copy/rename detection or other deltas that might want a special
indicator.

While making this change, I officially moved some of the flags that
were internal only into the private diff header.

This also allowed me to remove a gross hack in rename/copy detect
code where I was overwriting the status field with an internal
value.