Require subst-x only at compile-time

* lisp/net/eww.el (subr-x): Require subr-x at compile-time because eww
uses string-trim.
* lisp/vc/vc.el (vc-region-history): New command.
(vc-print-log-internal): Use cl-some.
* lisp/vc/vc-git.el (vc-git-region-history): New function.
(vc-git-region-history-mode-map)
(vc-git--log-view-long-font-lock-keywords)
(vc-git-region-history-font-lock-keywords): New vars.
(vc-git-region-history-font-lock): New function.
(vc-git-region-history-mode): New major mode.