subrepo: ensure "lock.release()" execution at the end of "_cachestorehash()"
Before this patch, "lock.release()" for "self._repo" in
"_cachestorehash()" of "hgsubrepo" may not be executed, if unexpected
exception is raised, because it isn't executed in "finally" clause.
This patch ensures "lock.release()" execution at the end of
"_cachestorehash()" by moving it into "finally" clause.