git-svn: cleanup: put SVN workarounds into their own namespace

git-svn: cleanup: put SVN workarounds into their own namespace

Force some svn_ra functions to use a temporary pool via wrapper

This cleans up the code a bit by removing explicit instances of
pool allocation and deallocation and providing wrapper functions
that make use of temporary pools.

I've also added an explicit pool usage when creating the commit
editor for commit-diff where get_commit_editor can be called
multiple times with the same pool previously.

Signed-off-by: Eric Wong <normalperson@yhbt.net>