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`