rep-cache sanity check on commit

When the commit process finds a representation in the rep-cache the only
sanity check that happens is that the revision must be less than or
equal to HEAD. We don't check that the offset is valid:

 echo foo > foo
 svnadmin create repo
 svn import -mm foo file://`pwd`/repo/A/f
 sqlite3 repo/db/rep-cache.db "update rep_cache set offset = 4"
 svn import -mm foo file://`pwd`/repo/A/g

or that the checksum at that offset matches:

 echo foo > foo
 echo bar > bar
 svnadmin create repo
 svn import -mm foo file://`pwd`/repo/A/f
 sqlite3 repo/db/rep-cache.db "update rep_cache set hash='e242ed3bffccdf271b7fbaf34ed72d089537b42f'"
 svn import -mm bar file://`pwd`/repo/A/g

In both cases corruption in the rep-cache leads to corruption in the
revision files but that corruption is not detected by commit process
even though subsequent checkouts will fail.

Should we do more sanity checking? We are using rep-cache to discard
data supplied by the client on the basis that it is already present in
the repository. Should we check that the offset really is a representation
with the expected checksum?

A side issue: "svnadmin verify" doesn't detect the corruption in the
second case even though checkout will fail.

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