RFR: git-pr: use multi-command argument parser

[Erik Helin]

Hi all,

please review this refactoring of `git pr` to start to use the
`MultiCommandParser`. This refactoring is needed for several reasons:

- not all subcommands support all options
- better help messages
- better reuse of utility functions
- the code was essentially one gigantic `main` function

I plan to split this file into multiple files, one for each command, but that is
for a later refactoring. I have tried to preserve the semantics of all commands
and have tested them locally.

Thanks,
Erik

-------------

Commits:
 - 5a0abe42: Remove tabs
 - 52fe55e9: git-pr: use multi-command argument parser

Changes: https://git.openjdk.java.net/skara/pull/458/files
 Webrev: https://webrevs.openjdk.java.net/skara/458/webrev.00
  Stats: 1676 lines in 1 file changed: 790 ins; 575 del; 311 mod
  Patch: https://git.openjdk.java.net/skara/pull/458.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/458/head:pull/458

PR: https://git.openjdk.java.net/skara/pull/458