checkout -m: attempt merge when deletion of path was staged

twoway_merge() is missing an o->gently check in the case where a file
that needs to be modified is missing from the index but present in the
old and new trees.  As a result, in this case 'git checkout -m' errors
out instead of trying to perform a merge.

Fix it by checking o->gently.  While at it, inline the o->gently check
into reject_merge to prevent future call sites from making the same
mistake.

Noticed by code inspection.  The test for the motivating case was
added by JC.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7201-co.sh

@@ -223,6 +223,23 @@ test_expect_success 'checkout --merge --conflict=diff3 <branch>' '
 	test_cmp two expect
 '
 
+test_expect_success 'switch to another branch while carrying a deletion' '
+
+	git checkout -f master && git reset --hard && git clean -f &&
+	git rm two &&
+
+	test_must_fail git checkout simple 2>errs &&
+	test_i18ngrep overwritten errs &&
+
+	git checkout --merge simple 2>errs &&
+	test_i18ngrep ! overwritten errs &&
+	git ls-files -u &&
+	test_must_fail git cat-file -t :0:two &&
+	test "$(git cat-file -t :1:two)" = blob &&
+	test "$(git cat-file -t :2:two)" = blob &&
+	test_must_fail git cat-file -t :3:two
+'
+
 test_expect_success 'checkout to detach HEAD (with advice declined)' '
 
 	git config advice.detachedHead false &&

unpack-trees.c

@@ -1178,7 +1178,8 @@ return_failed:
 static int reject_merge(const struct cache_entry *ce,
 			struct unpack_trees_options *o)
 {
-	return add_rejected_path(o, ERROR_WOULD_OVERWRITE, ce->name);
+	return o->gently ? -1 :
+		add_rejected_path(o, ERROR_WOULD_OVERWRITE, ce->name);
 }
 
 static int same(const struct cache_entry *a, const struct cache_entry *b)
@@ -1633,7 +1634,7 @@ int threeway_merge(const struct cache_entry * const *stages,
 	/* #14, #14ALT, #2ALT */
 	if (remote && !df_conflict_head && head_match && !remote_match) {
 		if (index && !same(index, remote) && !same(index, head))
-			return o->gently ? -1 : reject_merge(index, o);
+			return reject_merge(index, o);
 		return merged_entry(remote, index, o);
 	}
 	/*
@@ -1641,7 +1642,7 @@ int threeway_merge(const struct cache_entry * const *stages,
 	 * make sure that it matches head.
 	 */
 	if (index && !same(index, head))
-		return o->gently ? -1 : reject_merge(index, o);
+		return reject_merge(index, o);
 
 	if (head) {
 		/* #5ALT, #15 */
@@ -1770,7 +1771,7 @@ int twoway_merge(const struct cache_entry * const *src,
 				else
 					return merged_entry(newtree, current, o);
 			}
-			return o->gently ? -1 : reject_merge(current, o);
+			return reject_merge(current, o);
 		} else if ((!oldtree && !newtree) || /* 4 and 5 */
 			 (!oldtree && newtree &&
 			  same(current, newtree)) || /* 6 and 7 */
@@ -1788,7 +1789,7 @@ int twoway_merge(const struct cache_entry * const *src,
 			/* 20 or 21 */
 			return merged_entry(newtree, current, o);
 		} else
-			return o->gently ? -1 : reject_merge(current, o);
+			return reject_merge(current, o);
 	}
 	else if (newtree) {
 		if (oldtree && !o->initial_checkout) {