Git/SVN: die when there is no commit metadata

Git/SVN: die when there is no commit metadata

When passing a bad --trunk option to `git svn clone`, like for example the
same URL that we are cloning:

  C:\Windows\system32>git svn clone
  https://mycompany.svn.beanstalkapp.com/myproject --no-metadata -A
  c:\temp\svn_to_git_users.txt
  --trunk=https://mycompany.svn.beanstalkapp.com/myproject
  --tags=https://mycompany.svn.beanstalkapp.com/myproject/tags
  --branches=https://mycompany.svn.beanstalkapp.com/myproject/branches
  c:\code\Git_myproject

One gets an "Use of uninitialized value $u in substitution (s///)" error:

  [...]
  W: +empty_dir: branches/20080918_DBDEPLOY/vendor/src/csharp/MS WCSF
  Contrib/src/Services
  W: +empty_dir: branches/20080918_DBDEPLOY/vendor/src/csharp/RealWorldControls/References
  r530 = c276e3b039d8e38759c6fb17443349732552d7a2 (refs/remotes/origin/trunk)
  Found possible branch point:
  https://mycompany.svn.beanstalkapp.com/myproject/trunk =>
  https://mycompany.svn.beanstalkapp.com/myproject/branches/20080918_DBDEPLOY,
  529
  Use of uninitialized value $u in substitution (s///) at
  /mingw32/share/perl5/site_perl/Git/SVN.pm line 101.
  Use of uninitialized value $u in concatenation (.) or string at
  /mingw32/share/perl5/site_perl/Git/SVN.pm line 101.
  refs/remotes/origin/trunk:
  'https://mycompany.svn.beanstalkapp.com/myproject' not found in ''
  C:\Windows\system32>

Let's fix that by just die()ing when we have an uninitialized value because we
cannot get commit metadata from a ref.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Eric Wong <e@80x24.org>