[neomutt-devel] GitHub Merging and Squashing

[Richard Russon]

WIP: https://www.neomutt.org/run/merge

Recently, someone asked me the difference between GitHub's three merge options.
Here are some helpful pictures:


          A---B---C topic
         /
    D---E---F---G master

A developer has created branch from master's commit E.
(this is known as a topic branch in git terminology)
They committed A, B, C.

Meanwhile commits F and G were added to master.

------------------------------------------------------------------------

Create a Merge Commit
---------------------

          A---B---C topic
         /
    D---E---F---G master

git checkout master
git merge --no-ff topic

          A---B---C topic
         /         \
    D---E---F---G---H master

The topic branch has been merged into master with a "merge commit".
The topic's history hasn't been changed.

Squash and Merge
----------------

          A---B---C topic
         /
    D---E---F---G---H master

git checkout topic
git rebase master

                  A---B---C topic
                 /
    D---E---F---G master

The user's changes have been reapplied to the HEAD of master.

git rebase --interactive

                  H topic
                 /
    D---E---F---G master

Git's interactive rebase allows you to reorder commits and squash them
into one bit commit: H.

git checkout master
git merge --ff topic

    D---E---F---G---H master

The squashed commit, H, has been added to the HEAD of master.

Rebase and Merge
----------------

          A---B---C topic
         /
    D---E---F---G master

git rebase master

The user's changes have been reapplied to the HEAD of master.

                  A---B---C topic
                 /
    D---E---F---G master

git checkout master
git merge --no-ff topic

                  A---B---C topic
                 /         \
    D---E---F---G-----------H master

The topic branch has been merged into master with a "merge commit".  The
topic's history looks the same, but appears to have happened just now.

This option makes the history look tidiers.  Rather than having multiple
topic branches being created and merged at different times (all
overlapping), they are separated out.

Here are three topic branches being merged sequentially:

                  o---o---o      o---o---o      o---o---o
                 /         \    /         \    /         \
    o---o---o---o-----------o--o-----------o--o-----------o master

------------------------------------------------------------------------

When to merge?  When to squash?

When work is merged into master:

- Each commit should build
- Each commit should be self-contained

If there are 1 or 2 commits, then fast-forward master:
  `merge --ff`

If there are more than 2 commits, then rebase and merge into master:
  `merge --no-ff`

GitHub's documentation is here:
  https://help.github.com/articles/merging-a-pull-request/
  https://help.github.com/articles/about-pull-request-merges/

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