This is a major reorganization of the diff code. This changes
the diff functions to use the iterators for traversing the
content. This allowed a lot of code to be simplified. Also,
this moved the functions relating to outputting a diff into a
new file (diff_output.c).
This includes a number of other changes - adding utility
functions, extending iterators, etc. plus more tests for the
diff code. This also takes the example diff.c program much
further in terms of emulating git-diff command line options.
* Implemented git_diff_index_to_tree
* Reworked git_diff_options structure to handle more options
* Made most of the options in git_diff_options actually work
* Reorganized code a bit to remove some redundancy
* Added option parsing to examples/diff.c to test most options
This fixes several bugs, updates tests and docs, eliminates the
FILE* assumption in favor of printing callbacks for the diff patch
formatter helpers, and adds a "diff" example function that can
perform a diff from the command line.
