b4 doesn't notice updated base-commit

Hi, just noticed this minor bug in b4 v0.8.0: when fetching the latest version of a series, b4 seems to always advise that the base-commit is whatever was advertised in the first cover letter, even if the base-commit changes for later versions of the series. An example (from the Git list):

```
$ b4 am -o - [email protected] | git am
Looking up https://lore.kernel.org/r/pull.1161.v4.git.1647843442911.gitgitgadget%40gmail.com
[...]
 Base: dab1b7905d0b295f1acef9785bb2b9cbb0fdec84
       git checkout -b v4_20220228_gitgitgadget_gmail_com dab1b7905d0b295f1acef9785bb2b9cbb0fdec84
```

However, v4 of that series doesn't apply cleanly on dab1b79, and if we check the actual v4 message we see that it has a new base-commit of 74cc1aa55f30ed76424a0e7226ab519aa6265061.

Thanks,
Josh

I can trigger the issue on my end too, but I suspect it's more specific than "always [advising] that the base-commit is whatever was advertised in the first cover letter".

For example, here's a thread that has a different base commit at v1 and the latest version (v10), and the correct base is picked up (b80121027d12 not 5d01301f2b86):

https://lore.kernel.org/git/[email protected]/

Perhaps the problem with the thread you referenced has something to do with v4 consisting of only one patch and no cover letter while the previous iterations had multiple patches and a cover letter.

The cover letter written by 'b4 am' suggests it's getting stuck on the cover letter for the v3:

  $ b4 am [email protected]
  $ ls -1
  v4_20220228_gitgitgadget_adding_new_branch_autosetupmerge_option_simple.cover
  v4_20220228_gitgitgadget_adding_new_branch_autosetupmerge_option_simple.mbx
  $ grep Subject v4_20220228_gitgitgadget_adding_new_branch_autosetupmerge_option_simple.cover
  Subject: [PATCH v3 0/2] adding new branch.autosetupmerge option "simple"