t9118 (git-svn): prevent early failure from taking down later tests

t9118 (git-svn): prevent early failure from taking down later tests

When test #2 fails, the cwd is project/, causing all the
remaining tests in the same script to get confused and fail.

So in the spirit of v1.7.1.1~53^2~10 (t5550-http-fetch: Use subshell
for repository operations, 2010-04-17), use a subshell for svn
working copy operations.  This way, the cwd will reliably return
to the top of the trash directory and later tests can still be run
when a command has failed.

Reported-by: A Large Angry SCM <gitzilla@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Eric Wong <normalperson@yhbt.net>