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
Added test to check executable bit removal.