Add remote flag to `executable-find`

`git-gutter:in-git-repository-p` and `git-gutter:in-repository-common-p` must check whether an executable command exists on the remote environment while the buffer file is opened via tramp, so added the second argument of `remote` flag to `executable-find`. See also https://github.com/emacsorphanage/git-gutter/pull/231#issuecomment-2076205496