perf(git): cache index blob sha, drop rev-parse from the edit path

lua/git/core/repo.lua

@@ -123,6 +123,16 @@ local function affects_resolve(relpath)
         or relpath == "FETCH_HEAD"
 end
 
+---@private
+---@param prefix string
+function Repo:_clear_cache_prefix(prefix)
+    for key in pairs(self._cache) do
+        if vim.startswith(key, prefix) then
+            self._cache[key] = nil
+        end
+    end
+end
+
 ---@private
 ---@param relpath string
 function Repo:_invalidate(relpath)
@@ -132,11 +142,10 @@ function Repo:_invalidate(relpath)
         end
     end
     if affects_resolve(relpath) then
-        for key in pairs(self._cache) do
+        self:_clear_cache_prefix("resolve:")
-            if vim.startswith(key, "resolve:") then
-                self._cache[key] = nil
-            end
     end
+    if relpath == "index" then
+        self:_clear_cache_prefix("index:")
     end
 end
 
@@ -587,6 +596,15 @@ function Repo:rev_parse(rev, short)
     return trimmed ~= "" and trimmed or nil
 end
 
+---@param rel string  worktree-relative path
+---@return string?
+function Repo:index_sha(rel)
+    local sha = self:get_cached("index:" .. rel, function(self)
+        return self:rev_parse(":" .. rel, false) or false
+    end)
+    return sha or nil
+end
+
 ---@alias ow.Git.Repo.ResolveStatus "ok"|"ambiguous"|"missing"
 
 ---@param abbrev string

lua/git/hunks.lua

@@ -262,8 +262,7 @@ local function recompute(buf)
     if not state then
         return
     end
-    local r = state.repo
+    local new_sha = state.repo:index_sha(state.rel)
-    local new_sha = r:rev_parse(":" .. state.rel, true)
     if not new_sha then
         state.index = nil
         state.index_sha = nil

test/git/repo_test.lua

@@ -71,6 +71,32 @@ t.test("get_cached memoizes by key", function()
     t.truthy(v1 == v2, "second call should return cached table")
 end)
 
+t.test("index_sha returns the blob sha and caches it", function()
+    local dir = h.make_repo({ a = "x\n" })
+    local r = assert(require("git.core.repo").resolve(dir))
+    local sha = r:index_sha("a")
+    t.truthy(sha and #sha > 0, "index_sha returns the stage-0 blob sha")
+    t.truthy(r._cache["index:a"] ~= nil, "the result is cached")
+    t.eq(r:index_sha("a"), sha, "a cached call returns the same sha")
+end)
+
+t.test("index_sha caches a negative result for an untracked path", function()
+    local dir = h.make_repo({ a = "x\n" })
+    local r = assert(require("git.core.repo").resolve(dir))
+    t.eq(r:index_sha("nope"), nil, "an untracked path has no index sha")
+    t.eq(r._cache["index:nope"], false, "the negative result is cached")
+end)
+
+t.test("index_sha cache clears when the index is written", function()
+    local dir = h.make_repo({ a = "x\n" })
+    local r = assert(require("git.core.repo").resolve(dir))
+    r:index_sha("a")
+    t.truthy(r._cache["index:a"] ~= nil, "sha is cached before the stage")
+    t.write(dir, "a", "y\n")
+    h.git(dir, "add", "a")
+    wait_cleared(r, "index:a", 2000)
+end)
+
 t.test("cache clears after top-level .git change (commit)", function()
     local dir = h.make_repo({ a = "x" })
     local r = assert(require("git.core.repo").resolve(dir))