zsh:alias:git-base-branch: Match merge commits too

Merge commits should also be matched (indicated by a '-') for the case
that the base branches last commit is a merge commit and another branch
exists that checked out the newest commit that was merged.

Example log (from `glog`):
```
* abcdef Foo (HEAD -> feature)
*   bcdefa Merge branch 'other-feature' into base (base)
|\
| * cdefab Bar (other-feature)
|/
* ...
...
```

with `git show-branch -a` output like:
```
! [base] Merge branch 'other-feature' into base
 * [feature] Foo
  ! [other-feature] Bar
---
 *  [feature] Foo
--  [base] Merge branch 'other-feature' into base
+*+ [other-feature] Bar
```

Here we want `base` to be returned instead of `other-feature` and thus
'-' needs to be matched as well.

.config/zsh/zshrc.d/30-alias.zsh

@@ -34,7 +34,7 @@
 	alias gss='git stash'
 	# https://nilansanjaya.wordpress.com/2017/06/02/git-find-base-branch/
 	git_bb='git show-branch -a 2>/dev/null'
-	git_bb+=' | grep "^[^[]*\*"'
+	git_bb+=' | grep "^[^[]*[*-].*\["'
 	git_bb+=' | grep -v "^[^[]*\[$(git rev-parse --abbrev-ref HEAD)\]"'
 	git_bb+=' | head -n1'
 	git_bb+=' | sed -E "s/^[^[]*\[([^]~^]*).*$/\1/"'