Both git and Mercurial are distributed version control systems. While with svn, the repository was stored on a server and all you had was a working copy, with DVCSes everybody has their own repository. This means that you can make local changes on your computer, commit them to your local repository to build a history, then you can push your changes to a central repository when you're ready to share them. You could also keep using git or Mercurial for your hack by committing your changes to your local repository without ever pushing them to the central repository. By committing regularly, you keep a version history and don't need to make manual back-ups in case you screw up with your hack; however, you'd still have to back up your repository or push it to a remote location in case your hard disk fails. (Technically, it would have been possible to use a local svn repository, but not with Retro's svn repository history.) Also, by building your hack in a repository, you can later pull changes made to Retro's repository and integrate them in your hack; if you edit very little code in your hack, the changes will most of the time be merged automatically.
And hopefully, the web access clients for git and Mercurial don't suck as much as WebSVN. :P
To help you make a choice between git and Mercurial, should you be asking yourself that question, here are some notes about them:
- git is faster than Mercurial.
- On Windows, git doesn't handle characters outside of ASCII (characters 0 to 127, for those who think ASCII is more than that) correctly in file names and commit messages: it reads them using the system's ANSI code page (e.g. CP1252) but then tries to interpret the bytes as UTF-8, which fails. This means that such characters would probably be rendered as ? or �. Mercurial also has this problem, but it can be fixed for the most part by using the fixutf8 extension (the only issue is when you try to create or clone a repository in a directory that contains characters outside of ASCII, in which case the repository will not be created correctly).
EDIT: GerbilSoft made me notice utf8-git-on-windows, which aims to fix this problem.
- git identifies revisions using a SHA-1 hash. Unlike svn, revision identifiers are not sequential. This wouldn't make sense anyway, because it is possible to create revisions in separate repositories and merge them together later on. Mercurial does this too, but it also uses sequential revisions numbers. However, these sequential numbers are only relevant in a specific repository, so they won't necessarily make sense in other repositories, even if they're all based on the same source repository.
- Both have graphical clients on Windows. TortoiseGit is build from TortoiseSVN and tries to make git work like svn, which isn't necessarily a good thing... TortoiseHg, on the other hand, is completely different; it's really made for Mercurial. It's available as a Windows shell extension and as a GNOME/Nautilus extension.