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

"git diff" refused to even show difference when core.safecrlf is
set to true (i.e. error out) and there are offending lines in the
working tree files.

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

diff --cc diff.c
Simple merge