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

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>