[PATCH] Simplify and optimize RA-svn's string marshalling

Hi Stefan2 and others.

This patch simplifies code in RA-svn/marshal.c by using a single code
path to read both short and long strings efficiently. Using a single
code path is beneficial for test coverage.

It is an alternative to r1028352 which merged r985606 and r1028092 from
the performance branch.

I have run the test suite over RA-svn (with BDB) but haven't done any
more testing than that. It's just something I noticed while looking at
that code, it's not solving a problem I've noticed or anything like
that. By inspection I believe it is more efficient than
read_long_string() was, and unchanged for short strings, but if you're
able to do any performance analysis on it, that would be great.

- Julian