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When the user is asked to update oh-my-zsh it says "[Oh My Zsh] Would
you like to check for updates? [Y/n]:". When the user agreed to update
the next text would say "Upgrading Oh My Zsh" which is inconsistent
with the question.

@fcrozat's original fix assumes `which` not to output anything to STDOUT
in case the command is not found. That is not necessarily true on all
systems. A better solution is to check the return value instead.

Fixes #4376

Do not use terminal visual effects if not connected to a terminal.

After the upgrade is interesting to know what's new.

Changing the working directory in a sub-subshell
does not change the working directory of the executing
shell.

The substitution was broken for me on _all_ my machines,
so I started looking into the business.

If the current directory has spaces, the script fails to change paths and fails.

update.sh is an "sh" script, not a zsh or bash script. On platforms
which have real sh, echo does not have the -e option.

Make tools/install.sh and tools/upgrade.sh pretty.