fix(foreach): supports quoted commands

[jesusfcr]

This PR allows to execute foreach commands with parameters that include spaces.

turbolift foreach git commit -a -m "my commit that usually has spaces"
turbolift foreach sed -i 's/text with spaces/text-without-spaces/g' README.md

Also improves the code by reusing the same command slice for all the repositories.

[sledigabel]

Thanks for your contribution and great finding,
I left a small suggestion about the string builder.

Also could you rename the PR with something about what it fixes which is quoted commands in foreach?
Ideally something standard like

fix(foreach): supports quoted commands

[sledigabel]

LGTM now, but could you now add some unit tests around this fix?

[sledigabel]

LGTM, thank you for your contribution!

[annettejanewilson]

I think this breaks intended usages. Turbolift invokes a shell specifically to support invoking commands with pipes. So you can do things like this (slightly contrived).

turbolift foreach 'grep "foobar" README.md | sort`

If I understand correctly, this will cause that entire string to be quoted when passed to the shell, and then fail as an unrecognised command.

I do think that as it stood the handling for multiple arguments was a bit confused, but I'm worried this confuses things further. I think turbolift needs to sit on one side of this fence, and pick one of the following behaviours:

• It takes all the arguments from foreach unmodified, doesn't invoke a shell and just executes them as a command directly.
• It takes a single argument to foreach that is a shell script, and passes it to the shell via -c.

Some other concerns I have about this solution:

• I don't think it handles empty arguments. (E.g. printf '[%s]\n' 'one' '' 'three')
• I think it may have unexpected and confusing results if there are other characters that are special to the shell.
• I'm not convinced that golang's quoting rules match up with POSIX shell.

I'll have a look at how this behaves in practice and raise a ticket.