Update :/abc ambiguity check

:/abc may mean two things:

- as a revision, it means the revision that has "abc" in commit
  message.

- as a pathpec, it means "abc" from root.

Currently we see ":/abc" as a rev (most of the time), but never see it
as a pathspec even if "abc" exists and "git log :/abc" will gladly
take ":/abc" as rev even it's ambiguous. This patch makes it:

- ambiguous when "abc" exists on worktree
- a rev if abc does not exist on worktree
- a path if abc is not found in any commits (although better use
  "--" to avoid ambiguation because searching through commit DAG is
  expensive)

A plus from this patch is, because ":/" never matches anything as a
rev, it is never considered a valid rev and because root directory
always exists, ":/" is always unambiguously seen as a pathspec.

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

t/t4208-log-magic-pathspec.sh

@@ -11,11 +11,24 @@ test_expect_success 'setup' '
 	mkdir sub
 '
 
-test_expect_success '"git log :/" should be ambiguous' '
-	test_must_fail git log :/ 2>error &&
+test_expect_success '"git log :/" should not be ambiguous' '
+	git log :/
+'
+
+test_expect_success '"git log :/a" should be ambiguous (applied both rev and worktree)' '
+	: >a &&
+	test_must_fail git log :/a 2>error &&
 	grep ambiguous error
 '
 
+test_expect_success '"git log :/a -- " should not be ambiguous' '
+	git log :/a --
+'
+
+test_expect_success '"git log -- :/a" should not be ambiguous' '
+	git log -- :/a
+'
+
 test_expect_success '"git log :" should be ambiguous' '
 	test_must_fail git log : 2>error &&
 	grep ambiguous error