tag -v: use RUN_GIT_CMD to run verify-tag

tag -v: use RUN_GIT_CMD to run verify-tag

This is the preferred way to run a git command.

The only obvious observable effects I can think of are that the exec
is properly reported in GIT_TRACE output and that verifying signed
tags will still work if the git-verify-tag hard link in gitexecdir
goes missing.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>