Be more cautious in completing Eshell variable assignments

Be more cautious in completing Eshell variable assignments

Previously, Eshell treated cases like the second argument in "tar
--directory=dir" as a variable assignment, but that prevented
'pcomplete/tar' from implementing its own completion for that
argument (bug#61778).

* lisp/eshell/esh-var.el (eshell-complete-variable-assignment): Only
handle completion when all initial arguments are variable assignments.

* test/lisp/eshell/em-cmpl-tests.el
(em-cmpl-test/variable-assign-completion/non-assignment): New test.