allow for telling the difference between empty and non-existent files Adjust labels used in diff output accordingly; non-existent files should have the label "/dev/null"

add some API functions which allow access to opaque struct diff_chunk

add diff_chunk_context_load_change() for use with diff_output_unidiff_chunk()

revert db941c24; callers would also need private structs for that idea to work

put functions required for using diff_output_unidiff_chunk() into public API They will be needed to implement 'got stage -p' and 'got revert -p', etc. Chunks stored in diff_result still need post-processing in order to be displayed as expected by diff_output_unidiff_chunk(). Exposing these functions allows API consumers to implement the same chunk merging loop as used internally by output_unidiff() and output_edscript(). (Perhaps, ideally, the public diff API would provide pre-merged chunks at some point in the future? Then this commit could probably be reverted.) Change return values from bool to int to avoid making stdbool.h a requirement for the public diff API.

add a diff output mode which produces simple ed(1) scripts

move headers from include/diff/ to include/; makes things easier for consumers