shellcheck: cov-diff issues

Shellcheck (​​​https://github.com/koalaman/shellcheck) finds the following issues:

In cov-diff line 10:
for B in $DIRB/*; do
         ^-- SC2231: Quote expansions in this for loop glob to prevent wordsplitting, e.g. "$dir"/*.txt .


In cov-diff line 11:
  A=$DIRA/`basename $B`
          ^-- SC2006: Use $(..) instead of legacy `..`.
                    ^-- SC2086: Double quote to prevent globbing and word splitting.


In cov-diff line 12:
  if [ -f $A ]; then
          ^-- SC2086: Double quote to prevent globbing and word splitting.