For git commits, rebases etc I use jemacs as EDITOR. Can I use temacs
for such simple editing jobs?
If the concern is with startup time, then a cleaner solution is just
not to create a new emacs instance every time, and instead keep a
single emacs process running that you connect to every time git needs
to invoke an editor. In other words, start emacs just once, as a
daemon (or put an existing emacs process into server mode), and have
git invoke the editor as "emacsclient" rather than "emacs". See the
info node "(emacs) Emacs Server" for an overview of the various ways
one can use such a set up.