Improve handling of links in commit script

Changes introduced

For the commit script introduce the following changes

• allow name of link and name of linked file to be different
• allow the link to point to the same DSID directory

Tests

Fixes #111 (closed)

Issues resolved

Closes #111 (closed)

/label new feature

@cgutscho the first can be considered a bug fix. For the second if you think it shouldn't be done I could do instead:

if [[ $oldLinkedFile =~ .*/[0-9]{6}/.* ]] ; then
   # replace links 
else
  echo "ERROR: link pointing inside the same DSID directory"
  exit
fi