Fixes #962: First create a git_patch to get delta->flags correctly

[yoichi]

diff_get_delta_byindex() may result incorrect value of delta->flags
as described in the comment of git_diff_get_delta() (git2/diff.h)

Note that the flags on the delta related to whether it has binary content or not may not be set if there are no attributes set for the file and there has been no reason to load the file data at this point. For now, if you need those flags to be up to date, your only option is to either use `git_diff_foreach` or create a `git_patch`. 

this pull request fixes#962

[jdavid]

My concern is that building the patch would be a waste of time if the flags are not used later.
The alternative would be to build the patch lazily, when the flags are asked for, but this may not be easy.

I've done the related commit 464836c , if none of GIT_DIFF_FLAG_BINARY and GIT_DIFF_FLAG_NOT_BINARY have been set then the flags have not been loaded and the patch should be built.