This change sets the `GIT_DIFF_SHOW_BINARY` flag on all `git_diff`
structs created using `git_diff_from_buffer`. This ensures that when the
diff is printed (e.g. with `git_diff_print` or `git_diff_to_buf`) all
binary data patches in the diff are actually printed out.

Before this change printing a diff created from a patch file using
`git_diff_from_buffer` would result in binary data patches not being
included in the output. Instead, a "Binary files A and B differ" message
would be included in the printed patch, even if the original patch did
include a "GIT binary patch". This meant that you couldn't actually
round-trip such diffs, going from patch file to `git_diff` to patch
file.

Note that this change is consistent with how that same flag is already
set on all `git_patch` structs created using `git_patch_parse` (which
`git_diff_from_buffer` calls to populate the `git_diff`, and which
`git_patch_from_buffer` also calls).