Merge branch 'stash-p-corner-case'

This topic branch fixes a corner case that is amazingly common in this
developer's workflow: in a `git stash -p`, splitting a hunk and stashing
only part of it runs into a (known) bug where the partial hunk cannot be
applied in reverse.

It is one of those "good enough" fixes, not a full fix, though, as the
full fix would require a 3-way merge between `stash^` and the *worktree*
(not `HEAD`), with `stash` as merge base (i.e. a `git revert`, but on
top of the current worktree).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

builtin/stash.c

@@ -1036,7 +1036,7 @@ static int stash_patch(struct stash_info *info, const struct pathspec *ps,
 	}
 
 	cp_diff_tree.git_cmd = 1;
-	argv_array_pushl(&cp_diff_tree.args, "diff-tree", "-p", "HEAD",
+	argv_array_pushl(&cp_diff_tree.args, "diff-tree", "-p", "-U1", "HEAD",
 			 oid_to_hex(&info->w_tree), "--", NULL);
 	if (pipe_command(&cp_diff_tree, NULL, 0, out_patch, 0, NULL, 0)) {
 		ret = -1;

git-legacy-stash.sh

@@ -213,7 +213,7 @@ create_stash () {
 		w_tree=$(GIT_INDEX_FILE="$TMP-index" git write-tree) ||
 		die "$(gettext "Cannot save the current worktree state")"
 
-		git diff-tree -p HEAD $w_tree -- >"$TMP-patch" &&
+		git diff-tree -p -U1 HEAD $w_tree -- >"$TMP-patch" &&
 		test -s "$TMP-patch" ||
 		die "$(gettext "No changes selected")"
 

t/t3904-stash-patch.sh

@@ -89,7 +89,7 @@ test_expect_success 'none of this moved HEAD' '
 	verify_saved_head
 '
 
-test_expect_failure 'stash -p with split hunk' '
+test_expect_success 'stash -p with split hunk' '
 	git reset --hard &&
 	cat >test <<-\EOF &&
 	aaa
@@ -106,8 +106,8 @@ test_expect_failure 'stash -p with split hunk' '
 	ccc
 	EOF
 	printf "%s\n" s n y q |
-	test_might_fail git stash -p 2>error &&
-	! test_must_be_empty error &&
+	git stash -p 2>error &&
+	test_must_be_empty error &&
 	grep "added line 1" test &&
 	! grep "added line 2" test
 '