Follow-up to r1631171 & friends: Fix hash key lifetime issue.
Port the fix to all backends.

* subversion/libsvn_fs_base/bdb/changes-table.c
  (fold_change): Allocate the hash key for DELETIONS in said hash.

* subversion/libsvn_fs_fs/transaction.c
  (fold_change): Same.

* subversion/libsvn_fs_x/transaction.c
  (fold_change): Same.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632646 13f79535-47bb-0310-9956-ffa450edef68