Merge branch 'jc/maint-diff-core-safecrlf' into maint

Avoid failing "git diff" when core.safecrlf is set to true, because
the user cannot tell where the breakage is in preparation for fixing
and committing.

* jc/maint-diff-core-safecrlf:
diff: demote core.safecrlf=true to core.safecrlf=warn

diff --cc diff.c
Simple merge