RFR: token: handle arguments without scheme and add help message

[Erik Helin]

Hi all,

please review this patch that makes `git-token` print a more informative message when the user is about to enter the
token. Also made sure that `git token store <forge>.com` works by adding `https://` (there are currently no forges that
serves over plain HTTP).

Thanks,
Erik

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Commit messages:
 - token: handle arguments without scheme and add help message

Changes: https://git.openjdk.java.net/skara/pull/780/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=780&range=00
  Stats: 5 lines in 1 file changed: 5 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/780.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/780/head:pull/780

PR: https://git.openjdk.java.net/skara/pull/780