Fix shellcheck warnings

Fix the following warnings from shellcheck:

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In populate-fully.sh line 18:
  if kill -0 $(cat "$LOCKDIR/pid") 2>/dev/null;
             ^-- SC2046: Quote this to prevent word splitting.

In populate-sparsely.sh line 18:
. "$XDG_CONFIG_HOME/vcsh/hooks-available/sparse-checkout.sh"
^-- SC1090: Can't follow non-constant source. Use a directive to specify location.
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