message: build: check potential db migration conflict for open PRs
diff: @@ -1026,7 +1026,7 @@ Submissions will be considered for submission (or removal) on a case-by-case bas
  When two DB migrations collide, you'll get an error message like this one:
  -```
  +```text
  alembic.util.exc.CommandError: Multiple head revisions are present for
  given argument 'head'; please specify a specific target
  revision, '<branchname>@head' to narrow to a specific head,
  @@ -1041,15 +1041,46 @@ To fix it:
  superset db heads
  ```
  - This should list two or more migration hashes.
  + This should list two or more migration hashes. E.g.
  +
  + ```bash
  + 1412ec1e5a7b (head)
  + 67da9ef1ef9c (head)
  + ```
  +
  +2. Pick one of them as the parent revision, open the script for the other revision
  + and update `Revises` and `down_revision` to the new parent revision. E.g.:
  +
  + ```diff
  + --- a/67da9ef1ef9c_add_hide_left_bar_to_tabstate.py
  + +++ b/67da9ef1ef9c_add_hide_left_bar_to_tabstate.py
  + @@ -17,14 +17,14 @@
  + """add hide_left_bar to tabstate
  +
  + Revision ID: 67da9ef1ef9c
  + -Revises: c501b7c653a3
  + +Revises: 1412ec1e5a7b
  + Create Date: 2021-02-22 11:22:10.156942
  +
  + """
  +
  + # revision identifiers, used by Alembic.
  + revision = "67da9ef1ef9c"
  + -down_revision = "c501b7c653a3"
  + +down_revision = "1412ec1e5a7b"
  +
  + import sqlalchemy as sa
  + from alembic import op
  + ```
  -1. Create a new merge migration
  + Alternatively you may also run `superset db merge` to create a migration script
  + just for merging the heads.
  ```bash
  superset db merge {HASH1} {HASH2}
  ```
  -1. Upgrade the DB to the new checkpoint
  +3. Upgrade the DB to the new checkpoint
  ```bash
  superset db upgrade
  