Merge branch 'jc/diff-I-status-fix'

"git diff -I<pattern> -exit-code" should exit with 0 status when
all the changes match the ignored pattern, but it didn't.

* jc/diff-I-status-fix:
  diff: correct interaction between --exit-code and -I<pattern>

diff --cc diff.c
Simple merge

diff --cc t/t4015-diff-whitespace.sh
Simple merge