Add --rebase-merges to rebase commands

Closes #3179.

I'm not actually sure it's this simple; I'm just starting with a WIP MR (before any testing) in case:

• someone can tell me that it really is this simple; or
• the pipeline results point me to the ways in which it's not this simple.

Tested locally with GDK. I repeated the testing I described in #3179 (comment 422210268), and I noticed that the merge commits did not disappear upon clicking "Rebase".

Interestingly, it did appear that some commits were displayed in a different order than before - but the timestamps indicated that they were still properly ordered, as did the git log:

$ git log --graph --abbrev-commit --decorate --format=format:'%C(bold blue)%h%C(reset) - %C(bold cyan)%aD%C(reset) %C(bold green)(%ar)%C(reset)%C(bold yellow)%d%C(reset)%n''          %C(white)%s%C(reset) %C(dim white)- %an%C(reset)' 

*   fcc8a16 - Mon, 8 Mar 2021 02:13:55 +0000 (28 minutes ago) (HEAD -> shared-feature-dev, origin/shared-feature-dev)
|\            Merge branch 'root-shared-feature-dev-patch-12627' into 'shared-feature-dev' - Administrator
| * b7d7902 - Mon, 8 Mar 2021 02:13:38 +0000 (28 minutes ago)
|/            Implement subfeature C - Administrator
*   fbedadc - Mon, 8 Mar 2021 02:12:57 +0000 (29 minutes ago)
|\            Merge branch 'subfeature-b' into 'shared-feature-dev' - Administrator
| * a5bf8c8 - Mon, 8 Mar 2021 02:12:26 +0000 (29 minutes ago)
|/            Implement subfeature B - Administrator
*   91772f4 - Mon, 8 Mar 2021 02:11:34 +0000 (30 minutes ago)
|\            Merge branch 'subfeature-a' into 'shared-feature-dev' - Administrator
| * 58aa487 - Mon, 8 Mar 2021 02:11:09 +0000 (31 minutes ago)
|/            Implement subfeature A - Administrator
* eb0fc55 - Mon, 8 Mar 2021 02:20:26 +0000 (21 minutes ago) (origin/master, origin/HEAD, master)