Add git_diff_patch_size() API

This adds a new API to get the size in bytes of the diffs in a
git_diff_patch object.
Add hunk/file headers to git_diff_patch_size

This allows git_diff_patch_size to account for hunk headers and
file headers in the returned size.  This required some refactoring
of the code that is used to print file headers so that it could be
invoked by the git_diff_patch_size API.

Also this increases the test coverage and fixes an off-by-one bug
in the size calculation when newline changes happen at the end of
the file.
Make compact output more like core Git
Restore GIT_DIFF_LINE_BINARY usage

This restores the usage of GIT_DIFF_LINE_BINARY for the diff
output line that reads "Binary files x and y differ" so that it
can be optionally colorized independently of the file header.
Merge pull request #1738 from libgit2/diff-patch-content-size

Add API for getting at git_diff_patch->content_size