feat: unified diff conflict resolution for unmerged files (#99)

## Problem

Pressing `du` on a `UU` (unmerged) file in the fugitive status buffer
had no
effect. There was no way to see a proper ours-vs-theirs diff with syntax
highlighting and intra-line changes, or to resolve conflicts from within
a
unified diff view.

Additionally, pressing `du` on a section header containing only unmerged
files
showed "no changes in section" because `git diff` produces combined
(`diff --cc`)
output for unmerged files, which was stripped entirely.

## Solution

Fetch `:2:` (ours) and `:3:` (theirs) from the git index and generate a
standard
unified diff. The existing highlight pipeline (treesitter + intra-line)
applies
automatically. Resolution keymaps (`doo`/`dot`/`dob`/`don`) on hunks in
the diff
view write changes back to the working file's conflict markers.
Navigation
(`]x`/`[x`) jumps between unresolved conflict hunks.

For section diffs, combined diff entries are now replaced with generated
ours-vs-theirs unified diffs instead of being stripped.

Works for merge, cherry-pick, and rebase conflicts — git populates
`:2:`/`:3:`
the same way for all three.

Closes #61

lua/diffs/fugitive.lua

@@ -27,19 +27,19 @@ function M.get_section_at_line(bufnr, lnum)
 end
 
 ---@param line string
----@return string?, string?
+---@return string?, string?, string?
 local function parse_file_line(line)
   local old, new = line:match('^R%d*%s+(.-)%s+->%s+(.+)$')
   if old and new then
-    return vim.trim(new), vim.trim(old)
+    return vim.trim(new), vim.trim(old), 'R'
   end
 
-  local filename = line:match('^[MADRCU?][MADRCU%s]*%s+(.+)$')
-  if filename then
-    return vim.trim(filename), nil
+  local status, filename = line:match('^([MADRCU?])[MADRCU%s]*%s+(.+)$')
+  if status and filename then
+    return vim.trim(filename), nil, status
   end
 
-  return nil, nil
+  return nil, nil, nil
 end
 
 ---@param line string
@@ -57,34 +57,34 @@ end
 
 ---@param bufnr integer
 ---@param lnum integer
----@return string?, diffs.FugitiveSection, boolean, string?
+---@return string?, diffs.FugitiveSection, boolean, string?, string?
 function M.get_file_at_line(bufnr, lnum)
   local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
   local current_line = lines[lnum]
 
   if not current_line then
-    return nil, nil, false, nil
+    return nil, nil, false, nil, nil
   end
 
   local section_header = parse_section_header(current_line)
   if section_header then
-    return nil, section_header, true, nil
+    return nil, section_header, true, nil, nil
   end
 
-  local filename, old_filename = parse_file_line(current_line)
+  local filename, old_filename, status = parse_file_line(current_line)
   if filename then
     local section = M.get_section_at_line(bufnr, lnum)
-    return filename, section, false, old_filename
+    return filename, section, false, old_filename, status
   end
 
   local prefix = current_line:sub(1, 1)
   if prefix == '+' or prefix == '-' or prefix == ' ' then
     for i = lnum - 1, 1, -1 do
       local prev_line = lines[i]
-      filename, old_filename = parse_file_line(prev_line)
+      filename, old_filename, status = parse_file_line(prev_line)
       if filename then
         local section = M.get_section_at_line(bufnr, i)
-        return filename, section, false, old_filename
+        return filename, section, false, old_filename, status
       end
       if prev_line:match('^%w+ %(') or prev_line == '' then
         break
@@ -92,7 +92,7 @@ function M.get_file_at_line(bufnr, lnum)
     end
   end
 
-  return nil, nil, false, nil
+  return nil, nil, false, nil, nil
 end
 
 ---@class diffs.HunkPosition
@@ -150,7 +150,7 @@ function M.diff_file_under_cursor(vertical)
   local bufnr = vim.api.nvim_get_current_buf()
   local lnum = vim.api.nvim_win_get_cursor(0)[1]
 
-  local filename, section, is_header, old_filename = M.get_file_at_line(bufnr, lnum)
+  local filename, section, is_header, old_filename, status = M.get_file_at_line(bufnr, lnum)
 
   local repo_root = get_repo_root_from_fugitive(bufnr)
   if not repo_root then
@@ -192,6 +192,7 @@ function M.diff_file_under_cursor(vertical)
     vertical = vertical,
     staged = section == 'staged',
     untracked = section == 'untracked',
+    unmerged = status == 'U',
     old_filepath = old_filepath,
     hunk_position = hunk_position,
   })