Fix commit retrieval by tag (#21804)

It is not correct to return tag data when commit data is requested, so
remove the hacky code that overwrote parts of a commit with parts of a
tag.

This fixes commit retrieval by tag for both the latest commit in the UI
and the commit info on tag webhook events.

Fixes: https://github.com/go-gitea/gitea/issues/21687
Replaces: https://github.com/go-gitea/gitea/pull/21693

<img width="324" alt="Screenshot 2022-11-13 at 15 26 37"
src="https://user-images.githubusercontent.com/115237/201526975-736c6ea7-ad6a-467a-a823-9a63d6ecb718.png">

<img width="789" alt="image"
src="https://user-images.githubusercontent.com/115237/201526876-90a13ffc-1e5c-4d76-911b-f1ae51e8eaab.png">

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/git/tests/repos/repo1_bare/logs/HEAD

@@ -1 +1,2 @@
 37991dec2c8e592043f47155ce4808d4580f9123 feaf4ba6bc635fec442f46ddd4512416ec43c2c2 silverwind <me@silverwind.io> 1563741799 +0200	push
+feaf4ba6bc635fec442f46ddd4512416ec43c2c2 ce064814f4a0d337b333e646ece456cd39fab612 silverwind <me@silverwind.io> 1668354026 +0100	push

modules/git/tests/repos/repo1_bare/logs/refs/heads/master

@@ -1 +1,2 @@
 37991dec2c8e592043f47155ce4808d4580f9123 feaf4ba6bc635fec442f46ddd4512416ec43c2c2 silverwind <me@silverwind.io> 1563741799 +0200	push
+feaf4ba6bc635fec442f46ddd4512416ec43c2c2 ce064814f4a0d337b333e646ece456cd39fab612 silverwind <me@silverwind.io> 1668354026 +0100	push

modules/git/tests/repos/repo1_bare/refs/heads/master

@@ -1 +1 @@
-feaf4ba6bc635fec442f46ddd4512416ec43c2c2
+ce064814f4a0d337b333e646ece456cd39fab612

modules/git/tests/repos/repo1_bare/refs/tags/signed-tag

@@ -0,0 +1 @@
+36f97d9a96457e2bab511db30fe2db03893ebc64