[git-llvm] Check if svn and git-svn have been installed.

[ruiu]

Summary

The error messages that git-llvm script prints out when svn or
git-svn are missing is very cryptic. I spent a fair amount of time
to find what was wrong with my environment. It looks like many
newcomers also exprienced a hard time to submit their first patches
due to this error.

This patch adds more user-friendly error messages.

[rnk]

llvm/utils/git-svn/git-llvm

136

Maybe call it program_exists? exists suggests that it is checking for file existence.

319–320

I don't think this script relies on git-svn, so this shouldn't be necessary.

[ruiu]

• Address review comments

[rnk]

lgtm

llvm/utils/git-svn/git-llvm

def git(*cmd, **kwargs):
     return shell(['git'] + list(cmd), kwargs.get('strip', True))
 
 
 def svn(cwd, *cmd, **kwargs):
     # TODO: Better way to do default arg when we have *cmd?
     return shell(['svn'] + list(cmd), cwd=cwd, stdin=kwargs.get('stdin', None),
                  ignore_errors=kwargs.get('ignore_errors', None))
 
+def program_exists(cmd):
+    for path in os.environ["PATH"].split(os.pathsep):
+        if os.access(os.path.join(path, cmd), os.X_OK):
+            return True
+    return False
 
 def get_default_rev_range():
     # Get the branch tracked by the current branch, as set by
     # git branch --set-upstream-to  See http://serverfault.com/a/352236/38694.
     cur_branch = git('rev-parse', '--symbolic-full-name', 'HEAD')
     upstream_branch = git('for-each-ref', '--format=%(upstream:short)',
                           cur_branch)
     if not upstream_branch:
log('Pushing %d commit%s:\n%s' %
          else '', '\n'.join('  ' + git('show', '--oneline', '--quiet', c)
                             for c in revs)))
     for r in revs:
         clean_and_update_svn(svn_root)
         svn_push_one_rev(svn_root, r, dry_run)
 
 
 if __name__ == '__main__':
+    if not program_exists('svn'):
+        die('error: git-llvm needs svn command, but svn is not installed.')
+
     argv = sys.argv[1:]
     p = argparse.ArgumentParser(
         prog='git llvm', formatter_class=argparse.RawDescriptionHelpFormatter,
         description=__doc__)
     subcommands = p.add_subparsers(title='subcommands',
                                    description='valid subcommands',
                                    help='additional help')
     verbosity_group = p.add_mutually_exclusive_group()
     verbosity_group.add_argument('-q', '--quiet', action='store_true',