Merge branch 'lm/add-p-context'

Merge branch 'lm/add-p-context'

"git add/etc -p" now honor the diff.context configuration variable,
and also they learn to honor the -U<n> command-line option.

* lm/add-p-context:
  add-patch: add diff.context command line overrides
  add-patch: respect diff.context configuration
  t: use test_config in t4055
  t: use test_grep in t3701 and t4055