Add content offset to git_diff_line

For additions and deletions, external consumers like subversion
can make use of the content offset to generate diffs in their
proprietary formats.
util: NetBSD doesn't have qsort_r either
Merge pull request #1970 from ghedo/netbsd_qsort

util: NetBSD doesn't have qsort_r either
Merge pull request #1966 from nickh/patch_content_offsets

Add content offset to git_diff_line