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.

test-drop.t
test-inhibit.t
test-simple4server.t
tests/test-simple4server-bundle2.t