feat(blame): show commit's hunk in the line popup

test/git/hunks_test.lua

@@ -608,6 +608,32 @@ t.test("preview_hunk shows the hunk body without file headers", function()
     end
 end)
 
+t.test("preview_hunk highlights diff lines via per-line extmarks", function()
+    local _, buf = setup("a\nb\nc\n", "a\nB\nc\n")
+    vim.api.nvim_set_current_buf(buf)
+    vim.api.nvim_win_set_cursor(0, { 2, 0 })
+    hunks.preview_hunk(buf)
+    local float = assert(find_float(), "preview float should open")
+    t.defer(function()
+        pcall(vim.api.nvim_win_close, float, true)
+    end)
+    local pbuf = vim.api.nvim_win_get_buf(float)
+    local seen = {} ---@type table<string, boolean>
+    for _, m in
+        ipairs(vim.api.nvim_buf_get_extmarks(pbuf, -1, 0, -1, {
+            details = true,
+        }))
+    do
+        local hl = m[4] and m[4].line_hl_group
+        if hl then
+            seen[hl] = true
+        end
+    end
+    t.truthy(seen["GitHunkHeader"], "@@ header gets GitHunkHeader")
+    t.truthy(seen["GitHunkAdded"], "+ line gets GitHunkAdded")
+    t.truthy(seen["GitHunkRemoved"], "- line gets GitHunkRemoved")
+end)
+
 t.test("preview_hunk re-invocation focuses the open float", function()
     local _, buf = setup("a\nb\nc\n", "a\nB\nc\n")
     vim.api.nvim_set_current_buf(buf)