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
-            if vim.startswith(key, "resolve:") then
-                self._cache[key] = nil
-            end
-        end
+        self:_clear_cache_prefix("resolve:")
+    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