fix(web): read the wiki blob excerpt from the wiki repository (#40165)

The "load more lines" button in wiki diffs returned 404. `ExcerptBlob`
loads the commit from the wiki repo but read the blob from
`ctx.Repo.GitRepo`, a regression from
https://github.com/go-gitea/gitea/pull/38464.

Read the blob from the repo the commit was loaded from.

---------

Co-authored-by: Abdel <hktitof@users.noreply.github.com>
Co-authored-by: silverwind <me@silverwind.io>

routers/web/repo/compare.go

@@ -736,7 +736,7 @@ func ExcerptBlob(ctx *context.Context) {
 		ctx.ServerError("GetCommit", err)
 		return
 	}
-	blob, err := commit.GetBlobByPath(ctx, ctx.Repo.GitRepo, filePath)
+	blob, err := commit.GetBlobByPath(ctx, gitRepo, filePath)
 	if err != nil {
 		ctx.ServerError("GetBlobByPath", err)
 		return

tests/integration/wiki_test.go

@@ -37,6 +37,9 @@ func TestRepoWikiPages(t *testing.T) {
 
 		assert.Equal(t, expectedPagePaths[i], pagePath)
 	})
+
+	req = NewRequest(t, "GET", "/user2/repo1/wiki/blob_excerpt/a5bbc0fd39a696feabed2d4cccaf05abbcaf3b02?last_left=0&last_right=0&left=2&right=2&left_hunk_size=2&right_hunk_size=2&path=Home.md&style=split&direction=up")
+	MakeRequest(t, req, http.StatusOK)
 }
 
 func testRepoWikiCloneHTTP(t *testing.T, u *url.URL) {