add timestamp_validator to tag --timestamp option

Dependencies

• [2] NS36CJCO Validating timestamps in record
• [3] QL6K2ZM3 Tags
• [4] AAXP2534 Tags: completing the subcommand
• [5] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).
• [6] 4H2XTVJ2 Fix some mistakes in the docs

Change contents

• edit in pijul/src/commands/tag.rs at line 5
  [3.150]

  [3.150]
  + use crate::commands::record::timestamp_validator;
• replacement in pijul/src/commands/tag.rs at line 36
  [3.1086]→[3.1086:1122](∅→∅)
  − #[clap(long = "timestamp")]
  [3.1086]

  [3.1122]
  + #[clap(long = "timestamp", validator = timestamp_validator)]
• replacement in pijul/src/commands/record.rs at line 57
  [4.102760]→[2.66:128](∅→∅)
  − fn timestamp_validator(s: &str) -> Result<(), &'static str> {
  [4.102760]

  [2.128]
  + pub(crate) fn timestamp_validator(s: &str) -> Result<(), &'static str> {