Copied over branches

Author: axiezai

_episodes/03-branches.md

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+---
+title: "Branching"
+teaching: 10
+exercises: 10
+questions:
+- "What is a branch?"
+- "How can I merge changes from another branch?"
+objectives:
+- "Know what branches are and why you would use them"
+- "Understand how to merge branches"
+- "Understand how to resolve conflicts during a merge"
+keypoints:
+- "`git branch` creates a new branch"
+- "Use feature branches for new ideas and fixes, before merging into `master`"
+- merging does not delete any branches
+---
+
+### What is a branch?
+
+You might have noticed the term *branch* in status messages:
+
+~~~
+$ git status
+~~~
+{: .language-bash}
+~~~
+On branch master
+nothing to commit (working directory clean)
+~~~
+{: .output}
+
+and when we wanted to get back to our most recent version of the repository, we
+used `git checkout master`.
+
+Not only can our repository store the changes made to files and directories, it
+can store multiple sets of these, which we can use and edit and update in
+parallel. Each of these sets, or parallel instances, is termed a `branch` and
+`master` is Git's default branch.
+
+A new branch can be created from any commit. Branches can also be *merged*
+together.
+
+### Why are branches useful?
+Suppose we've developed some software and now we want to
+try out some new ideas but we're not sure yet whether we'll keep them. We
+can then create a branch 'feature1' and keep our `master` branch clean. When
+we're done developing the feature and we are sure that we want to include it
+in our program, we can merge the feature branch with the `master` branch.
+This keeps all the work-in-progress separate from the `master` branch, which
+contains tested, working code.
+
+When we merge our feature branch with master git creates a new commit which
+contains merged files from master and feature1. After the merge we can continue
+developing. **The merged branch is not deleted.** We can continue developing (and
+making commits) in feature1 as well.
+
+
+### Branching in practice
+
+One of our colleagues wants to contribute to the paper but is not quite sure
+if it will actually make a publication. So it will be safer to create a branch
+and carry on working on this "experimental" version of the paper in a branch
+rather than in the master.
+
+~~~
+$ git checkout -b paperWJohn
+~~~
+{: .language-bash}
+~~~
+Switched to a new branch 'paperWJohn'
+~~~
+{: .output}
+
+We're going to change the title of the paper and update the author list (adding John Smith).
+However, before we get started it's a good practice to check that we're working
+on the right branch.
+
+~~~
+$ git branch			# Double check which branch we are working on
+~~~
+{: .language-bash}
+~~~
+  master
+* paperWJohn
+~~~
+{: .output}
+
+The * indicates which branch we're currently in. Now let's make the changes to the paper.
+
+~~~
+$ vim journal.md		# Change title and add co-author
+$ git add journal.md
+$ git commit			# "Modify title and add John as co-author"
+~~~
+{: .language-bash}
+
+If we now want to work in our `master` branch. We can switch back by using:
+
+~~~
+$ git checkout master
+~~~
+{: .language-bash}
+~~~
+Switched to branch 'master'
+~~~
+{: .output}