git-svn: fix ls-tree usage with dash-prefixed paths

git-svn: fix ls-tree usage with dash-prefixed paths

To find the blob object name given a tree and pathname, we were
incorrectly calling "git ls-tree" with a "--" argument followed
by the pathname of the file we wanted to get.

  git ls-tree <TREE> -- --dashed/path/name.c

Unlike many command-line interfaces, the "--" alone does not
symbolize the end of non-option arguments on the command-line.

ls-tree interprets the "--" as a prefix to match against, thus
the entire contents of the --dashed/* hierarchy would be
returned because the "--" matches "--dashed" and every path
under it.

Thanks to Anton Gyllenberg for pointing me toward the
Twisted repository as a real-world example of this case.

Signed-off-by: Eric Wong <normalperson@yhbt.net>