fetch2: Honour BB_FETCH_PREMIRRORONLY option
    
    This should fix [Yocto 13353] and related to [Yocto 13233] as well.
    Previously if git repo mirror has been updated in between of two builds
    fetcher of the second build didn't try updated mirror but switched to
    git clone from upstream instead. This is problem for offline builds.
    Fix this to raise MirrorException if BB_FETCH_PREMIRRORONLY has been
    specified by the mirror doesn't contain SRC_REV.
    
    [YOCTO #13353]
    [YOCTO #13233]
    
    Signed-off-by: Pavel Zhukov <pavel.zhukov@huawei.com>
    Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
    Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>