git-update-dirs: Accept multiple -E/--exec-before and -e/--exec-after

Now it's possible to use -E and -e several times instead of having to
stick with things like

  git update-dirs -r -e "command 1; command 2; command 3"

This can now be written as

  git update-dirs -r -e "command 1" -e "command 2" -e "command 3"

The advantage of this is that the script can display the "Executing ..."
message before every command.

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