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

spec/commands_spec.lua

@@ -40,6 +40,78 @@ describe('commands', function()
     end)
   end)
 
+  describe('filter_combined_diffs', function()
+    it('strips diff --cc entries entirely', function()
+      local lines = {
+        'diff --cc main.lua',
+        'index d13ab94,b113aee..0000000',
+        '--- a/main.lua',
+        '+++ b/main.lua',
+        '@@@ -1,7 -1,7 +1,11 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+        ' +  return 1',
+        '++=======',
+        '+   return 2',
+        '++>>>>>>> theirs',
+        '  end',
+      }
+      local result = commands.filter_combined_diffs(lines)
+      assert.are.equal(0, #result)
+    end)
+
+    it('preserves diff --git entries', function()
+      local lines = {
+        'diff --git a/file.lua b/file.lua',
+        '--- a/file.lua',
+        '+++ b/file.lua',
+        '@@ -1,3 +1,3 @@',
+        ' local M = {}',
+        '-local x = 1',
+        '+local x = 2',
+        ' return M',
+      }
+      local result = commands.filter_combined_diffs(lines)
+      assert.are.equal(8, #result)
+      assert.are.same(lines, result)
+    end)
+
+    it('strips combined but keeps unified in mixed output', function()
+      local lines = {
+        'diff --cc conflict.lua',
+        'index aaa,bbb..000',
+        '@@@ -1,1 -1,1 +1,5 @@@',
+        '++<<<<<<< HEAD',
+        'diff --git a/clean.lua b/clean.lua',
+        '--- a/clean.lua',
+        '+++ b/clean.lua',
+        '@@ -1,1 +1,1 @@',
+        '-old',
+        '+new',
+      }
+      local result = commands.filter_combined_diffs(lines)
+      assert.are.equal(6, #result)
+      assert.are.equal('diff --git a/clean.lua b/clean.lua', result[1])
+      assert.are.equal('+new', result[6])
+    end)
+
+    it('returns empty for empty input', function()
+      local result = commands.filter_combined_diffs({})
+      assert.are.equal(0, #result)
+    end)
+
+    it('returns empty when all entries are combined', function()
+      local lines = {
+        'diff --cc a.lua',
+        'some content',
+        'diff --cc b.lua',
+        'more content',
+      }
+      local result = commands.filter_combined_diffs(lines)
+      assert.are.equal(0, #result)
+    end)
+  end)
+
   describe('find_hunk_line', function()
     it('finds matching @@ header and returns target line', function()
       local diff_lines = {