feat: add merge command

[armcknight]

In my personal workflow, I like to rebase as long as I'm working privately, but after getting reviews on PRs, I typically will only merge in upstream changes, to avoid breaking the history of the PR page. So, I would have use for a merge command that cascades merges up a chain. Here's how it looked after a brief test:

❯ glog
*   4b0d7cc (HEAD -> test-chain/3) Merge branch 'test-chain/2' into test-chain/3
|\
| *   0ccb636 (test-chain/2) Merge branch 'test-chain/1' into test-chain/2
| |\
| | * 66941c4 (test-chain/1) test chain edit 1b
* | | cac2d95 test chain edit 3
|/ /
* / fd17589 test chain edit 2
|/
* 2802e5d test chain edit 1
* ad28b75 (fork/fork_master, fork_master) fix alias instructions

[armcknight]

Added in c346bb4