git-svn: just name the default svn-remote "svn" instead of "git-svn"

It can be confusing and redundant, since historically the
default remote ref (not remote itself) has been "git-svn", too.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
diff --git a/git-svn.perl b/git-svn.perl
index 8c24012..66b4c20 100755
--- a/git-svn.perl
+++ b/git-svn.perl
@@ -10,7 +10,7 @@
 $VERSION = '@@GIT_VERSION@@';
 
 $ENV{GIT_DIR} ||= '.git';
-$Git::SVN::default_repo_id = 'git-svn';
+$Git::SVN::default_repo_id = 'svn';
 $Git::SVN::default_ref_id = $ENV{GIT_SVN_ID} || 'git-svn';
 
 $Git::SVN::Log::TZ = $ENV{TZ};