and a (failing) test for it.

This adds an option to refresh the stat cache while generating
status.  It also rips out the GIT_PERF stuff I had an makes use
of the trace API to keep statistics about what happens during diff.

and make tests empty on platforms without iconv support.

The basic structure of each function is courtesy of arrbee.

This changes `git_index_read` to have two modes - a hard index
reload that always resets the index to match the on-disk data
(which was the old behavior) and a soft index reload that uses
the timestamp / file size information and only replaces the index
data if the file on disk has been modified.

This then updates the git_status code to do a soft reload unless
the new GIT_STATUS_OPT_NO_REFRESH flag is passed in.

This also changes the behavior of the git_diff functions that use
the index so that when an index is not explicitly passed in (i.e.
when the functions call git_repository_index for you), they will
also do a soft reload for you.

This intentionally breaks the file signature of git_index_read
because there has been some confusion about the behavior previously
and it seems like all existing uses of the API should probably be
examined to select the desired behavior.