Fix gitlab shell test preparation reset method

[cirosantilli]

Bug introduced by self at: #7823

Before this PR, the test gitlab shell under tmp/tests/gitlab-shell will not correctly fetch new commits if necessary when it exists already (the cache was introduced on the PR that causes this problem: before it would always get removed and cloned).

This happens because the shell.rake sh reset line I wrote was completely wrong because:

1. bad understanding of git reset --hard: if there is no reference, reset --hard $() will do just git reset --hard, which is valid, and does not activate the ||
2. bad understanding of precedence: true || echo a && echo b translates to (true || echo a) && echo b, so the fetch part was never done!

Now it works, but is a monster expression. We could split this up into multiple system calls, but that would break the current convention of using sh.

This was extracted from: #8086

[TeatroIO]

I've prepared a stage. Click to open.

[jvanbaarsen]

@cirosantilli Can you please make this mergeable?

[jvanbaarsen]

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