perf(git): bounded upward search in show.diff_section

lua/git/show.lua

@@ -26,22 +26,21 @@ local function context()
     return { worktree = worktree, ref = ref, parent_ref = vim.b.git_parent_ref }
 end
 
----Walk upward from the cursor to the enclosing `diff --git` line and parse
----the section's pre/post paths plus the pre/post blob SHAs from the `index`
----line.
----@param cursor_lnum integer  1-indexed
+---Find the enclosing `diff --git` line and parse the section's pre/post
+---paths plus the pre/post blob SHAs from the `index` line.
+---
+---Uses `vim.fn.search('bcnW')` (backward, accept cursor pos, no move, no
+---wrap) so a giant `git show <merge>` buffer doesn't pay an O(cursor_lnum)
+---array allocation on every <CR>.
 ---@return ow.Git.DiffSection?
-local function diff_section(cursor_lnum)
-    local lines = vim.api.nvim_buf_get_lines(0, 0, cursor_lnum, false)
-    local diff_lnum, diff_line
-    for i = #lines, 1, -1 do
-        if lines[i]:match("^diff %-%-git ") then
-            diff_lnum = i
-            diff_line = lines[i]
-            break
-        end
+local function diff_section()
+    local diff_lnum = vim.fn.search("^diff --git ", "bcnW")
+    if diff_lnum == 0 then
+        return nil
     end
-    if not diff_lnum or not diff_line then
+    local diff_line =
+        vim.api.nvim_buf_get_lines(0, diff_lnum - 1, diff_lnum, false)[1]
+    if not diff_line then
         return nil
     end
     local pre_path, post_path = diff_line:match("^diff %-%-git a/(.-) b/(.+)$")
@@ -49,6 +48,9 @@ local function diff_section(cursor_lnum)
         return nil
     end
 
+    -- Header lines (mode/index/oldfile/newfile/etc) sit between the
+    -- `diff --git` line and the first `@@` hunk; cap the read at 20 to
+    -- bound work even for unusual diff headers.
     local header =
         vim.api.nvim_buf_get_lines(0, diff_lnum, diff_lnum + 20, false)
     local pre_blob, post_blob
@@ -165,8 +167,7 @@ function M.open_at_cursor()
     if not ctx then
         return false
     end
-    local cursor_lnum = vim.api.nvim_win_get_cursor(0)[1]
-    local section = diff_section(cursor_lnum)
+    local section = diff_section()
     if not section then
         return false
     end