[git-llvm] Only use --force-interactive when supported

The --force-interactive option was introduced in SVN 1.8, and trying to
pass it to older SVN clients causes an error; CentOS 7 includes SVN 1.7,
for example, so this makes `git llvm` not usable out of the box. Older
clients would be interactive by default anyway [1], so just don't pass
the option if it's not supported.

An alternative would be to check the version instead of checking the
help text, but I think directly detecting the presence of the option is
more direct.

[1] http://svn.apache.org/viewvc?view=revision&revision=1424037

Differential Revision: https://reviews.llvm.org/D59161

llvm-svn: 355782