If a vendor repo is removed under one name and re-added under a
different name, *both* names would be listed and accepted as
vendor names.

For example:
  $ git vendor add mispeled http://some/url
  $ git vendor remove mispeled
  $ git vendor add correct http://some/url
  $ git vendor list
  correct:
  mispeled:

To fix, validate each vendor name by identifying the most recent
vendor name associated with the subtree directory.

"git vendor -h" produces an unhelpful message:

  $ git vendor -h
  usage: git vendor

Move some code so a proper usage message will be issued.

Note that "git vendor --help" is processed by the "git" binary (not
"git-vendor") and thus is unaffected by this change.

support repositories with SHH URL.
e.g. given repository URL `ssh://git@some.domain.com:7999/project/cool-stuff.git`
before the change the resulting directory will be: `vendor/git@some.domain.com/7999/project/cool-stuff`
the directory after this change will be: `                 `vendor/some.domain.com/project/cool-stuff``

I have a feeling this script was created on OSX because then it would be
being executed using on old bash in strict mode. On Ubuntu 14.04 though
it's executed using dash and various parts fall over because there's a
bunch of bash-specific code in here.

I corrected the problems that the checkbashisms script reported and it
seems to be working better now.

- Use printf instead of echo where escapes are necessary.
- Don't use double bracketed if statements where unnecessary.
- Use -o instead of ||

fixes #4