feat: treesitter highlighting for diff headers

Apply treesitter highlighting to diff metadata lines (diff --git, index,
---, +++) using the diff language parser. Header info is attached only
to the first hunk of each file to avoid duplicate highlighting.

Based on PR #52 by @phanen with fixes:
- header_lines now only contains diff metadata, not hunk content
- header info attached only to first hunk per file
- removed arbitrary hunk count restriction

lua/diffs/parser.lua

@@ -6,6 +6,8 @@
 ---@field header_context string?
 ---@field header_context_col integer?
 ---@field lines string[]
+---@field header_start_line integer?
+---@field header_lines string[]?
 
 local M = {}
 
@@ -58,10 +60,16 @@ function M.parse_buffer(bufnr)
   local hunk_header_context_col = nil
   ---@type string[]
   local hunk_lines = {}
+  ---@type integer?
+  local hunk_count = nil
+  ---@type integer?
+  local header_start = nil
+  ---@type string[]
+  local header_lines = {}
 
   local function flush_hunk()
     if hunk_start and #hunk_lines > 0 and (current_lang or current_ft) then
-      table.insert(hunks, {
+      local hunk = {
         filename = current_filename,
         ft = current_ft,
         lang = current_lang,
@@ -69,7 +77,12 @@ function M.parse_buffer(bufnr)
         header_context = hunk_header_context,
         header_context_col = hunk_header_context_col,
         lines = hunk_lines,
-      })
+      }
+      if hunk_count == 1 and header_start and #header_lines > 0 then
+        hunk.header_start_line = header_start
+        hunk.header_lines = header_lines
+      end
+      table.insert(hunks, hunk)
     end
     hunk_start = nil
     hunk_header_context = nil
@@ -89,6 +102,9 @@ function M.parse_buffer(bufnr)
       elseif current_ft then
         dbg('file: %s -> ft: %s (no ts parser)', filename, current_ft)
       end
+      hunk_count = 0
+      header_start = i
+      header_lines = {}
     elseif line:match('^@@.-@@') then
       flush_hunk()
       hunk_start = i
@@ -97,6 +113,9 @@ function M.parse_buffer(bufnr)
         hunk_header_context = context
         hunk_header_context_col = #prefix
       end
+      if hunk_count then
+        hunk_count = hunk_count + 1
+      end
     elseif hunk_start then
       local prefix = line:sub(1, 1)
       if prefix == ' ' or prefix == '+' or prefix == '-' then
@@ -112,8 +131,12 @@ function M.parse_buffer(bufnr)
         current_filename = nil
         current_ft = nil
         current_lang = nil
+        header_start = nil
       end
     end
+    if header_start and not hunk_start then
+      table.insert(header_lines, line)
+    end
   end
 
   flush_hunk()