Let's say I have these commits in chronological order:

1. a
2. b
3. c

Now, I want to get rid of b but keep c, so that I have:

1. a
2. c

How do I do this?

(there are no conflicts between b and c)

• git

In case you already pushed the changes to a remote, you could use:

$ git revert <hash of commit b>

that creates a new commit d that removes the changes of commit b

Assuming you haven't already pushed to a remote repository, you can do an interactive rebase. See here:

http://book.git-scm.com/4_interactive_rebasing.html

If you only need the one commit from the HEAD, you can use cherry-pick on a different branch to bring just that commit over:

$ git checkout -b working
$ git reset --hard <hash of the commit `a`>
$ git cherry-pick <hash of the commit `c`>

The hard reset changes the working copy back to the state as of that commit, and then the cherry-pick applies the changes introduced in commit c directly on top of your working copy.

The help for git rebase talks about this exact case! Check it out:

A range of commits could also be removed with rebase. If we have the
   following situation:

           E---F---G---H---I---J  topicA

   then the command

       git rebase --onto topicA~5 topicA~3 topicA

   would result in the removal of commits F and G:

           E---H'---I'---J'  topicA

   This is useful if F and G were flawed in some way, or should not be
   part of topicA. Note that the argument to --onto and the <upstream>
   parameter can be any valid commit-ish.