Return a clean error instead of a panic when the channel for a commit is not found in `pijul git`

Dependencies

• [2] IIV3EL2X Cleanup, formatting, and fixing the Git feature
• [3] UCQD3JDH Fix build errors caused by sanakirja 0.14
• [4] A3DMBJJA Upgrading the `git` subcommand to the latest Sanakirja and Libpijul
• [5] SXEYMYF7 Fixing the bad changes in history (unfortunately, by rebooting).

Change contents

• replacement in pijul/src/commands/git.rs at line 239
  [3.144237]→[3.101:162](∅→∅)
  − let channel = txn.load_channel(&oid_)?.unwrap();
  [3.144237]

  [2.976]
  + let channel = if let Some(c) = txn.load_channel(&oid_)? {

  + c

  + } else {

  + bail!("Channel not found: {:?}", oid);

  + };