convert: handle exec bit removal while converting to svn Subversion `putcommit` method checks original file's executablity to decide if executable property should be removed from svn. It is checked right after writing file contents. Content writing is implemented using `vfs.write` and vfs seems to remove exec bit, at least in some cases. This leads to executability checks being ineffective. If cset contains only this ignored exec bit removal, conversion stops with an error, because it fails to to compose svn commit properly. This fix moves exec bit checking so that it's performed before dumping file contents. Added test to check executable bit removal.