feat(git/repo): add Repo:head_sha for cached HEAD blob lookups

lua/git/core/repo.lua

@@ -143,6 +143,7 @@ function Repo:_invalidate(relpath)
     end
     if affects_resolve(relpath) then
         self:_clear_cache_prefix("resolve:")
+        self:_clear_cache_prefix("head_blob:")
     end
     if relpath == "index" then
         self:_clear_cache_prefix("index:")
@@ -605,6 +606,15 @@ function Repo:index_sha(rel)
     return sha or nil
 end
 
+---@param rel string  worktree-relative path
+---@return string?
+function Repo:head_sha(rel)
+    local sha = self:get_cached("head_blob:" .. rel, function(self)
+        return self:rev_parse("HEAD:" .. rel, false) or false
+    end)
+    return sha or nil
+end
+
 ---@alias ow.Git.Repo.ResolveStatus "ok"|"ambiguous"|"missing"
 
 ---@param abbrev string