git-svn: fix bottleneck in stash_placeholder_list()

.git/config is written on each commit. It's slow
git-svn: fix empty dir tracking in branches

  - When creating a tag or branch from a subdir, a disjoint branch is
    created. Then git-svn re-imports the commits using this dir as strip
    path.

    During this re-import the variable %added_placeholder is not up to
    date. Because the branch is disjoint, this variable should be empty
    in the beginning, but it's not. Because of that git-svn tries to
    delete non-existent .gitignore files and dies.

  - When creating a tag or branch from a subdir, the strip path is e.g.
    "trunk/module", but change_dir_prop() can be called with just
    "trunk". This breaks tracking of placeholder files, because it
    relise on the hash {dir_prop}, filled in change_dir_prop().

  - When creating a normal tag or branch, git-svn creates a normal
    branch without reimport, but the placeholder files in the new
    branch are not added to %added_placeholder.

This patch does 3 things:

  - It makes git-svn store paths in %added_placeholder already
    translated from "trunk/subdir/" to "tags/subdir_1.0/" during
    reimport.

  - When strip path is "trunk/subdir", don't add "trunk" to {dir_prop}
    in change_dir_prop().

  - When a normal branch is created, it takes entries in
    %added_placeholder belonging to the source branch, translates them
    to target branch and adds them to %added_placeholder.