subrepo/svn: abort on commit with missing file (
issue3029)
Previous code was printing a traceback because it expected some error output
from svn. But sometimes our definition of "changed" differs with the subversion
one. For instance, subversion ignores missing files when committing. And when
there are only missing files, svn commit will be a successful no-op with no
output. Still, we should stick to our definition including missing files in
changes as doing otherwise could cause surprising behaviour for the user.