Merge branch 'dl/checkout-p-merge-base'

"git checkout -p A...B [-- <path>]" did not work, even though the
same command without "-p" correctly used the merge-base between
commits A and B.

* dl/checkout-p-merge-base:
  t2016: add a NEEDSWORK about the PERL prerequisite
  add-patch: add NEEDSWORK about comparing commits
  Doc: document "A...B" form for <tree-ish> in checkout and switch
  builtin/checkout: fix `git checkout -p HEAD...` bug

git-add--interactive.perl

@@ -1830,6 +1830,13 @@ sub process_args {
 				$arg = shift @ARGV or die __("missing --");
 				if ($arg ne '--') {
 					$patch_mode_revision = $arg;
+
+					# NEEDSWORK: Instead of comparing to the literal "HEAD",
+					# compare the commit objects instead so that other ways of
+					# saying the same thing (such as "@") are also handled
+					# appropriately.
+					#
+					# This applies to the cases below too.
 					$patch_mode = ($arg eq 'HEAD' ?
 						       'reset_head' : 'reset_nothead');
 					$arg = shift @ARGV or die __("missing --");