Merge branch 'ag/git-svn-global-ignores'

"git svn" has been taught about svn:global-ignores property
recent versions of Subversion has.

* ag/git-svn-global-ignores:
  git-svn: mention `svn:global-ignores` in help+docs
  git-svn: use `svn:global-ignores` to create .gitignore
  git-svn: add public property `svn:global-ignores`