fetch2: Don't allow '/' in user:pass, fix branch containing '@'

* currently decode_url regexp parses branch=@foo as username so it ends like this:
  - ('git', '', 'foo', 'git.openembedded.org/bitbake;branch=', '', {})
  + ('git', 'git.openembedded.org', '/bitbake', '', '', {'branch': '@foo'})
* http://hg.python.org/cpython/file/2.7/Lib/urlparse.py also assumes
  that there is at least one '/' as separator between netloc and path,
  params, so it looks reasonable to prevent including '/' in username

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
fetch2/gitsm: Use ud.basecmd instead of hardcoding git

This allows FETCHCMD_git to override the fetcher command as the git fetcher does.

[YOCTO #5717]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>