obslog: add header to the changeset description diff
This patch adds `diff -r <hash> -r <hash> <filename>` header to the output of
the changeset description diff which is shown using the --patch flag of obslog
command.
This fixes the label/color issue for description diff also.
syntax: re
/figures/[^/]+\.png$
^html/
\.pyc$
~$
\.swp$
\.orig$
\.rej$
^tests/.*\.err$
^tests/easy_run.sh$
^build/
^dist/
^MANIFEST$
^docs/tutorials/.*\.rst$
\.ico$
tests/\.testtimes
^docs/training/graphs/
^docs/training/html/
^docs/training/index.html
^docs/training/graphviz-images/
^docs/training/img/
^docs/training/output/
^docs/training/pandocfilters/