What are the benefits of Mercurial or git over svn for branching/merging?

Question

I've heard for instance that merging branches with git or mercurial is easier than with svn.

Reading last Joel on software blog entry, I didn't get it exactly why. Could you provide a concrete example where merging with git/mercurial lead to less merge conflicts compared to svn please?

Answer 1

One simple example is git can automatically convert a merge into a "fast forward". For example, let's say I have a branch that looks like this:

Master:

A ---> B ---> C

And I create a feature branch based on Master with new commits D and E.

Feature:

A --- > B ---> C
                \
                 D ---> E

In svn, when you merge the feature branch back into master, you must create an entirely new commit that applies the changes of D and E on Master. So, it looks like:

Master:
    A ---> B ---> C -----------------> F
Feature:           \                  /
                    ---> D ---> E -->

In git we have a choice of how to incorporate the branch feature into master. If we do a

git rebase feature

git will automatically recognize that this is a trivial merge and perform a fast-forward merge, which will add the new commits to the master branch. The result of the rebase is:

Master:

A ---> B ---> C ---> D ---> E

Both the head of Master and Feature point at commit E (in other words, they look exactly the same). A fast-forward merge is similar to what happens when you do an update in svn.

Additionally, you have the option of forcing git to create a merge commit. If instead we do:

git merge feature

git will create a merge commit. The result of the merge is:

Master:
    A ---> B ---> C -----------------> F
Feature:           \                  /
                    ---> D ---> E -->

Commit F is the combination of D and E.