isValidRefName: Inline the forbidden ref suffix of ".lock"

A Git reference name must never end with ".lock", as it would
confuse any existing C client that tries to obtain a clone of the
repository over the network.  Even if the repository isn't on a
local filesystem, it still should ban that suffix.

Because I plan to move LockFile to storage.file and make it a private
implementation detail of the local file system storage model,
we can't rely on its package level SUFFIX field here.  Making it
public probably won't work long-term either, as I also plan to
pull storage.file into its own separate project that depends on
the core library.

So, just inline the constant here.  Its as foribidden as ":" is.

Change-Id: If85076861baeacc183b82696375a13e935ba8836
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>