Add SVN compatibility

[michaelmcd18]

No description provided.

[andersone1]

@michaelmcd18

Can we:

1. Rename demoRepo -> demoRepoGit

2. Rename with_demoRepo -> with_demoRepoGit

3. Pull the code below out diffQced and put it into it's own function vcsExport:

if (vcs == "git") {

    # Prepend "./" so that 'git cat-file' interprets the path relative to the
    # working directory rather than the top-level directory of the Git repo.
    commit_file_new <- paste0(version_new, ":./", file_rel)
    commit_file_qc <- paste0(version_qc, ":./", file_rel)

    processx::run(
      "git", c("cat-file", "blob", commit_file_new),
      stdout = tempfile_new, wd = logDir())
    processx::run(
      "git", c("cat-file", "blob", commit_file_qc),
      stdout = tempfile_qc, wd = logDir())
  }

  if (vcs == "svn") {

    processx::run("svn", c("export", file_rel, paste0("-r", version_new), tempfile_new),
                  wd = logDir())

    processx::run("svn", c("export", file_rel, paste0("-r", version_qc), tempfile_qc),
                  wd = logDir())

  }