t/t91XX-svn: start removing use of "git-" from these tests

Subversion tests use too many "git-foo" form, so I am converting them
in two steps.

This first step replaces literal strings "remotes/git-svn" and "git-svn-id"
by introducing $remotes_git_svn and $git_svn_id constants defined as shell
variables.  This will reduce the number of false hits from "git grep".

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9123-git-svn-rebuild-with-rewriteroot.sh

@@ -17,7 +17,7 @@ rm -rf import
 test_expect_success 'init, fetch and checkout repository' '
 	git svn init --rewrite-root=http://invalid.invalid/ "$svnrepo" &&
 	git svn fetch
-	git checkout -b mybranch remotes/git-svn
+	git checkout -b mybranch ${remotes_git_svn}
 	'
 
 test_expect_success 'remove rev_map' '