Fix calling parse_pathspec with no paths nor PATHSPEC_PREFER_* flags

When parse_pathspec() is called with no paths, the behavior could be
either return no paths, or return one path that is cwd. Some commands
do the former, some the latter. parse_pathspec() itself does not make
either the default and requires the caller to specify either flag if
it may run into this situation.

I've grep'd through all parse_pathspec() call sites. Some pass
neither, but those are guaranteed never pass empty path to
parse_pathspec(). There are two call sites that may pass empty path
and are fixed with this patch.

[jc: added a test from Antoine's bug report]

Reported-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-log.c

@@ -747,7 +747,8 @@ void line_log_init(struct rev_info *rev, const char *prefix, struct string_list
 			r = r->next;
 		}
 		paths[count] = NULL;
-		parse_pathspec(&rev->diffopt.pathspec, 0, 0, "", paths);
+		parse_pathspec(&rev->diffopt.pathspec, 0,
+			       PATHSPEC_PREFER_FULL, "", paths);
 		free(paths);
 	}
 }

revision.c

@@ -1372,7 +1372,8 @@ static void prepare_show_merge(struct rev_info *revs)
 			i++;
 	}
 	free_pathspec(&revs->prune_data);
-	parse_pathspec(&revs->prune_data, PATHSPEC_ALL_MAGIC, 0, "", prune);
+	parse_pathspec(&revs->prune_data, PATHSPEC_ALL_MAGIC,
+		       PATHSPEC_PREFER_FULL, "", prune);
 	revs->limited = 1;
 }
 

t/t4208-log-magic-pathspec.sh

@@ -46,4 +46,19 @@ test_expect_success 'git log HEAD -- :/' '
 	test_cmp expected actual
 '
 
+test_expect_success 'command line pathspec parsing for "git log"' '
+	git reset --hard &&
+	>a &&
+	git add a &&
+	git commit -m "add an empty a" --allow-empty &&
+	echo 1 >a &&
+	git commit -a -m "update a to 1" &&
+	git checkout HEAD^ &&
+	echo 2 >a &&
+	git commit -a -m "update a to 2" &&
+	test_must_fail git merge master &&
+	git add a &&
+	git log --merge -- a
+'
+
 test_done