"git diff --no-such-option" and other corner cases around the exit
status of the "diff" command has been corrected.
* jk/diff-result-code-cleanup:
diff: drop useless "status" parameter from diff_result_code()
diff: drop useless return values in git-diff helpers
diff: drop useless return from run_diff_{files,index} functions
diff: die when failing to read index in git-diff builtin
diff: show usage for unknown builtin_diff_files() options
diff-files: avoid negative exit value
diff: spell DIFF_INDEX_CACHED out when calling run_diff_index()