feat: Add support for git worktrees

[kamilwaz]

I experimented a bit with worktrees and bare repositories and found a bug where Gitu isn’t able to fetch the history for a worktree created in a bare repository. This PR fixes the issue and makes the tests use the same function to open repositories as regular Gitu.

Error:

-old snapshot
+new results
────────────┬────────────────────────────────────────────────────────────────────────────────────────────────
    1     1 │ ▌On branch main                                                                 |
    2     2 │ ▌Your branch is up to date with 'origin/main'.                                  |
    3     3 │                                                                                 |
    4     4 │  Recent commits                                                                 |
    5       │- Couldn't read log: object not found - no match for id (b66a0bf82020d6a386e94d0…|
          5 │+ 46c81ca main new-branch origin/main add new-file                               |
    6     6 │                                                                                 |
    7     7 │                                                                                 |
    8     8 │                                                                                 |
    9     9 │                                                                                 |
┈┈┈┈┈┈┈┈┈┈┈┈┼┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
   17    17 │                                                                                 |
   18    18 │                                                                                 |
   19    19 │                                                                                 |
   20    20 │                                                                                 |
   21       │-styles_hash: 2b21a6aa63fe13b3
         21 │+styles_hash: 4d5a9a1a08dc1d60
────────────┴────────────────────────────────────────────────────────────────────────────────────────────────

Checklist

• The modified code has some test-coverage (where applicable).
• make test is passing (this is what CI runs).
• New unreleased features/fixes/styling and performance improvements are documented via git: feat: / fix: / style: / perf:. Or e.g. perf(highlighting):.
  See https://github.com/altsem/gitu/blob/master/docs/dev-tooling.md

[kamilwaz]

I replaced open_from_env, as GIT_DIR should already be handled by git rev-parse --show-toplevel.