perf: apply stage ops directly

When staging hunks, the cache would be fully invalidated which would
cause the next update to re-read the file from git.

Now the stage is applied directly to the cache and is now only partially
validated. This should result in the signs updating faster.

Note the index watcher will still eventually trigger and cause a full
invalidate and update.

Author: lewis6991

lua/gitsigns/actions.lua

@@ -273,10 +273,15 @@ function M.stage_hunk(range, opts, callback)
         message.error(err)
         return
       end
+
+      if bcache.compare_text then
+        bcache.compare_text = Hunks.apply_to_text(bcache.compare_text, hunk, invert)
+      end
+
       table.insert(bcache.staged_diffs, hunk)
     end)
 
-    bcache:invalidate(true)
+    bcache:invalidate()
     update(bufnr)
   end)
 end
@@ -438,11 +443,14 @@ function M.stage_buffer(callback)
       end
 
       for _, hunk in ipairs(hunks) do
+        if bcache.compare_text then
+          bcache.compare_text = Hunks.apply_to_text(bcache.compare_text, hunk)
+        end
         table.insert(bcache.staged_diffs, hunk)
       end
     end)
 
-    bcache:invalidate(true)
+    bcache:invalidate()
     update(bufnr)
   end)
 end

lua/gitsigns/hunks.lua

@@ -125,6 +125,33 @@ function M.patch_lines(hunk, fileformat)
   return lines
 end
 
+--- @param text string[]
+--- @param hunk Gitsigns.Hunk.Hunk
+--- @param reverse? boolean
+--- @return string[]
+function M.apply_to_text(text, hunk, reverse)
+  local removed = reverse and hunk.added or hunk.removed
+  local added = reverse and hunk.removed or hunk.added
+
+  local start = removed.start
+  if start == 0 then
+    start = 1
+  end
+
+  local new = {} --- @type string[]
+  if start > 1 then
+    vim.list_extend(new, vim.list_slice(text, 1, start - 1))
+  end
+  if added.count > 0 then
+    vim.list_extend(new, added.lines)
+  end
+  local tail_start = start + removed.count
+  if tail_start <= #text then
+    vim.list_extend(new, vim.list_slice(text, tail_start, #text))
+  end
+  return new
+end
+
 local function tointeger(x)
   return tonumber(x) --[[@as integer]]
 end