Support git-revise through merge commits

[MartyLake]

Hello,

I realise that my message #131 (comment) would better be fit for its own issue ticket.

I work on multiple feature branches, and usually keep them all together via a merge commit at the top, named all-feats.
During the day, I pile up commits on the all-feats branch, and then regularly redistribute them to their own branches, using the --update-refs option when rebasing..

I wish I could do that using git-revise, since rebasing across merge commits is very slow using git.

Here are some reproduction steps:

reproduction steps

$ mkdir /tmp/test-git-revise
$ cd /tmp/test-git-revise/
$ git init
Initialized empty Git repository in /private/tmp/test-git-revise/.git/
$ echo "lorem ipsum" > README
$ git add README
$ git config user.email "[email protected]"
$ git config user.name "example"
$ git commit -m "first commit"
[master (root-commit) ddeb30d] first commit
 1 file changed, 1 insertion(+)
 create mode 100644 README
$ git checkout -b feat-a
Switched to a new branch 'feat-a'
$ echo "lorem ipsum with a typo" > a.txt
$ # Here I forgot to add the new file and commit, it happens a bit later
$ git checkout master
Switched to branch 'master'
$ git checkout -b feat-b
Switched to a new branch 'feat-b'
$ echo "lorem ipsum" > b.txt
$ git add b.txt
$ git commit -m "featb"
[feat-b d22a5ea] featb
 1 file changed, 1 insertion(+)
 create mode 100644 b.txt
$ git checkout feat-a
Switched to branch 'feat-a'
$ git add a.txt
$ git commit -m "feata"
[feat-a 2628ddc] feata
 1 file changed, 1 insertion(+)
 create mode 100644 a.txt
$ git checkout -b "all-feats"
Switched to a new branch 'all-feats'
$ git merge feat-b
Merge made by the 'ort' strategy.
 b.txt | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 b.txt
$ echo "explain historic typo" >> a.txt
$ git add a.txt
$ git commit --fixup=2628ddc991de547b666047ec39e2fe28148
[all-feats 99e6e3c] fixup! feata
 1 file changed, 1 insertion(+)
$ git log --graph --oneline
* 99e6e3c (HEAD -> all-feats) fixup! feata
*   7a8e7af Merge branch 'feat-b' into all-feats
|\
| * d22a5ea (feat-b) featb
* | 2628ddc (feat-a) feata
|/
* ddeb30d (master) first commit

actual

$ git revise -i master
invalid value: Commit 7a8e7af951094fd1badedd9f52f507b7924901d9 has 2 parents

expected

$ git revise -i master

# git-revise displays an edit-todo similar to the edit-todo of `git rebase -i master` here