In vc-git-log-incoming use the repository in the command "fetch" (bug#50340)

In vc-git-log-incoming use the repository in the command "fetch" (bug#50340)

* lisp/vc/vc-git.el (vc-git-log-incoming): Use repository part
from non-nil remote-location in the command "fetch".

* lisp/vc/vc.el (vc-log-incoming, vc-log-outgoing): Mention in
docstrings and prompts that REMOTE-LOCATION can be a remote branch name.

* doc/emacs/maintaining.texi (VC Change Log): For commands
vc-log-incoming and vc-log-outgoing, use the term "remote location"
instead of "repository".