* lisp/tab-line.el: Fix auto-hscrolling (bug#39649)

* lisp/tab-line.el: Fix auto-hscrolling (bug#39649)

Distinguish offsets between manual-vs-automatic scrolling
as integers-vs-floats instead of positive-vs-negative integers.

* lisp/tab-line.el (tab-line-format-template): Use 'numberp'
instead of 'integerp', and 'truncate' instead of 'abs'.
(tab-line-format): When the window-buffer was updated, set window-parameter
to float to enable auto-hscroll after it was disabled on manual scrolling.
(tab-line-auto-hscroll-buffer): New variable with internal buffer.
(tab-line-auto-hscroll): Erase in tab-line-auto-hscroll-buffer.
Use 'numberp' instead of 'integerp', 'truncate' instead of 'abs',
and 'float' instead of '-'.
(tab-line-hscroll): Use 'numberp' instead of 'integerp',
and 'truncate' instead of 'abs'.