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

[smeenai]

Summary

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

llvm/utils/git-svn/git-llvm

for l in (l for l in status_lines if (l.startswith('?') or
                                           l.startswith('I'))):
         svn(svn_repo, 'add', '--no-ignore', l[1:].strip())
     for l in (l for l in status_lines if l.startswith('!')):
         svn(svn_repo, 'remove', l[1:].strip())
 
     # Now we're ready to commit.
     commit_msg = git('show', '--pretty=%B', '--quiet', rev)
     if not dry_run:
-        log(svn(svn_repo, 'commit', '-m', commit_msg, '--force-interactive'))
+        commit_args = ['commit', '-m', commit_msg]
+        if '--force-interactive' in svn(svn_repo, 'commit', '--help'):
+            commit_args.append('--force-interactive')
+        log(svn(svn_repo, *commit_args))
         log('Committed %s to svn.' % rev)
     else:
         log("Would have committed %s to svn, if this weren't a dry run." % rev)
 
 
 def cmd_push(args):
     '''Push changes back to SVN: this is extracted from Justin Lebar's script
     available here: https://github.com/jlebar/llvm-repo-tools/