Improved usage of local pools in `SubversionRepository._history`.
Improved usage of local pools in `SubversionRepository._history`.

This happens to be the same logic than the one used in the `svn_repos_history` function that [4710] replaced.

Should contribute to reduce the overall memory usage (related to #6614).
pkdevbox/trac,pkdevbox/trac,pkdevbox/trac,pkdevbox/trac