Introduce repo_read_object_file which takes the repository argument, and
hide the original read_object_file as a macro behind
NO_THE_REPOSITORY_COMPATIBILITY_MACROS, which we planned for in
e675765235 (diff.c: remove implicit dependency on the_index, 2018-09-21)

That commit didn't seem to plan for anything - it just seems to add a
new function with the name "repo_" preprended and define a macro if
NO_THE_REPOSITORY_COMPATIBILITY_MACROS is not set, just like this patch.
Maybe s/which we planned for in/just like in/.

The patch itself looks good.