refs: allow @{n} to work with n-sized reflog

This sequence works

	$ git checkout -b newbranch
	$ git commit --allow-empty -m one
	$ git show -s newbranch@{1}

and shows the state that was immediately after the newbranch was
created.

But then if you do

	$ git reflog expire --expire=now refs/heads/newbranch
	$ git commit --allow=empty -m two
	$ git show -s newbranch@{1}

you'd be scolded with

	fatal: log for 'newbranch' only has 1 entries

While it is true that it has only 1 entry, we have enough
information in that single entry that records the transition between
the state in which the tip of the branch was pointing at commit
'one' to the new commit 'two' built on it, so we should be able to
answer "what object newbranch was pointing at?". But we refuse to
do so.

Make @{0} the special case where we use the new side to look up that
entry. Otherwise, look up @{n} using the old side of the (n-1)th entry
of the reflog.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 15 insertions, 2 deletions

diff --git a/t/t1503-rev-parse-verify.sh b/t/t1503-rev-parse-verify.sh
index dc9fe3cbf1..a7e9b4863d 100755
--- a/t/t1503-rev-parse-verify.sh
+++ b/t/t1503-rev-parse-verify.sh
@@ -86,8 +86,8 @@ test_expect_success 'fails silently when using -q' '
 test_expect_success 'fails silently when using -q with deleted reflogs' '
 	ref=$(git rev-parse HEAD) &&
 	git update-ref --create-reflog -m "message for refs/test" refs/test "$ref" &&
-	git reflog delete --updateref --rewrite refs/test@{0} &&
-	test_must_fail git rev-parse -q --verify refs/test@{0} >error 2>&1 &&
+	git reflog delete --updateref --rewrite refs/test@{1} &&
+	test_must_fail git rev-parse -q --verify refs/test@{1} >error 2>&1 &&
 	test_must_be_empty error
 '
 
diff --git a/t/t1508-at-combinations.sh b/t/t1508-at-combinations.sh
index 4a9964e9dc..e4521b7b97 100755
--- a/t/t1508-at-combinations.sh
+++ b/t/t1508-at-combinations.sh
@@ -99,4 +99,17 @@ test_expect_success 'create path with @' '
 check "@:normal" blob content
 check "@:fun@ny" blob content
 
+test_expect_success '@{1} works with only one reflog entry' '
+	git checkout -B newbranch master &&
+	git reflog expire --expire=now refs/heads/newbranch &&
+	git commit --allow-empty -m "first after expiration" &&
+	test_cmp_rev newbranch~ newbranch@{1}
+'
+
+test_expect_success '@{0} works with empty reflog' '
+	git checkout -B newbranch master &&
+	git reflog expire --expire=now refs/heads/newbranch &&
+	test_cmp_rev newbranch newbranch@{0}
+'
+
 test_done