git-svn: problem with svn cp trunk/subdir tags/subdir_1.0

[Ilya Basin <basinilya@gmail.com>]

After fixing --preserve-empty-dirs plus --stdlayout a new problem
arised:
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.

Why? I would instead keep the current commit as parent, delete
everything except the subdir and move its contents to root directory.

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.

I think, if a disjoint branch is created, %added_placeholder should be
pushed and cleared. A new set of paths starting with "trunk/" will be
added to it during re-import.
When re-import is done, we should translate the paths to
"tags/subdir_1.0/" and merge with the original %added_placeholder.