Fix 'decomposedfs meta set' value handling

Don't strip off the first two characters of the target
value to set unconditionally. Also print an error if
decoding hex/base64 values fails (falling back to treating
the values as strings).
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f