v0.4.1: fix three real bugs in update.sh found by auditing it

Release-candidate tags would have been installed as stable
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git's version sort ranks v0.5.0-rc1 ABOVE v0.5.0 (verified empirically), and the
release workflow deliberately supports rc/beta/alpha tags. update.sh would have
offered an RC as "the newest release". Tag selection is now filtered to plain
vX.Y.Z.

update.sh would have died mid-update on an untracked file
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The dirty-tree guard uses --untracked-files=no, so an untracked file that the
TARGET tracks slips past it -- and `git checkout` then aborts. Under set -e the
script died with a raw git error, after already recording the rollback point.

Not hypothetical: a hand-copied patch/wait_restart.sh blocked a pull on a real box
in exactly this way. It is now detected up front, by name. Gitignored files are
correctly not treated as blockers, since git overwrites those silently.

Special case: if update.sh ITSELF is the blocker, it was hand-copied in to
bootstrap -- and "delete update.sh, then re-run update.sh" is impossible. It now
says so and prints the git commands that bootstrap it properly.

--rollback skipped that check entirely and would have hit the identical failure.
The check is now a shared function used by both paths, and rollback also validates
that the recorded revision still exists.

Also: install.sh's chmod aborted under set -e if a listed file was missing (the
file set changes between versions, so --rollback must not be killed by a name this
version happens to know about), and --to with no value was silently ignored.

Verified end to end in a throwaway clone: forward v0.4.1 -> v0.4.2 and rollback
back, with files appearing and disappearing correctly; both guards fire.

132 tests, ruff and shellcheck -S style clean.

recover.sh

@@ -17,7 +17,7 @@
 #   bash /mnt/tank/truenas-truecloud-patch/patch/apply.sh
 #   systemctl restart middlewared
 
-VERSION="0.4.0"
+VERSION="0.4.1"
 
 PATCH_DIR="$(cd "$(dirname "$0")" && pwd)"