Improve passing user switches to Git log commands (bug#59414)

Improve passing user switches to Git log commands (bug#59414)

* lisp/vc/vc-git.el (vc-git-log-switches): Revise docstring.
(vc-git-shortlog-switches): New defcustom.
(vc-git-print-log): Use vc-git-log-switches or
vc-git-shortlog-switches depending on whether printing a shortlog.
(vc-git-log-outgoing, vc-git-log-incoming): Use
vc-git-shortlog-switches.
(vc-git-log-search, vc-git-expanded-log-entry): Use
vc-git-log-switches.
* etc/NEWS: Document the new defcustom.