perf(git): skip sidebar render when porcelain output unchanged

lua/git/status_win.lua

@@ -37,6 +37,7 @@ local SIDEBAR_WIDTH = 50
 ---@field diff_right_win integer?
 ---@field user_aucmd integer?
 ---@field last_shown_key string?
+---@field last_render_key string?  fingerprint of the last rendered state
 
 ---@type table<integer, ow.Git.StatusState>
 local state = {}
@@ -328,10 +329,41 @@ local function render(bufnr, branch, groups)
         })
     end
     state[bufnr].lines = meta
-    -- The diff windows we last opened may now be showing content for an
-    -- entry whose underlying file has changed. Drop the cache so the next
-    -- show_diff recomputes pre/post panes instead of short-circuiting.
-    state[bufnr].last_shown_key = nil
+end
+
+---Build a stable fingerprint of the parsed branch + groups so refresh can
+---short-circuit when the porcelain state is byte-identical to the last
+---successful render.
+---@param branch ow.Git.BranchInfo
+---@param groups table<string, ow.Git.StatusEntry[]>
+---@return string
+local function fingerprint(branch, groups)
+    local parts = {
+        branch.head or "",
+        branch.upstream or "",
+        tostring(branch.ahead),
+        tostring(branch.behind),
+    }
+    for _, section in ipairs(SECTIONS) do
+        local entries = groups[section]
+        if entries then
+            for _, e in ipairs(entries) do
+                table.insert(
+                    parts,
+                    e.section
+                        .. ":"
+                        .. (e.path or e.sha or "")
+                        .. ":"
+                        .. (e.orig or "")
+                        .. ":"
+                        .. (e.x or "")
+                        .. ":"
+                        .. (e.y or "")
+                )
+            end
+        end
+    end
+    return table.concat(parts, "\0")
 end
 
 ---@param bufnr integer
@@ -356,6 +388,15 @@ local function refresh(bufnr)
         if not vim.api.nvim_buf_is_valid(bufnr) then
             return
         end
+        -- Any fs-event that triggered this refresh might have changed the
+        -- worktree under the diff buffers we last opened; invalidate the
+        -- cache so the next show_diff recomputes panes.
+        s.last_shown_key = nil
+        local fp = fingerprint(branch, groups)
+        if fp == s.last_render_key then
+            return
+        end
+        s.last_render_key = fp
         render(bufnr, branch, groups)
         if not saved_path and not saved_sha then
             return