Interpret a ${username} string in a ref as the name of the current
user. This allows the refs namespace to have generic rules applied
to refs involving the user's name without having to hardcode a rule
for each user.
Bug: issue 577
Change-Id: Ib8692a13221a4581731e753e9c7cccb7aa0a96da
Signed-off-by: Shawn O. Pearce <sop@google.com>
f14ada51fa