canReadCommit: move check for READ on refs/* on top

This should improve performance for those users who have READ access
on refs/*. Since this check is much cheaper than walking potentially
large commit graphs, move it to the top of the method. Only when user
doesn't have READ on refs/* do the usual reachability check from
all readable branches and tags.

Change-Id: I48341112347c1c4708c4b6b94c3326da6de8d8fb
1 file changed