Update repository_tool.py.

Properly strip (again) the digest prepended to 'digest.filename' files.
The required '+1' appears to have been accidentally deleted in a recent commit:
https://github.com/theupdateframework/tuf/commit/298f52ddb8a6c017e4892a2f1de6545f7715de59#diff-59d384d80d746c800b16c8387756c0ccL2750
Thanks to Santiago for locating the bug.
Merge branch 'repository-tools' of https://github.com/theupdateframework/tuf into repository-tools