You can clone with
HTTPS or Subversion.
Would you mind adding a portable .zip or .7z download for Win32 binary on the Sourceforge download page?
You could easily compile one build yourself described in Contributing to Tiled and provide it to other people - keep in mind though, that the editor is not "completely" portable as it writes some properties to the registry on Windows systems. (I don't know how this works under Mac OS and Linux)
Tiled currently adapts the way it stores settings to the operating system. For a portable build, you'd want to include some code that tells QSettings to use the .ini format and force it to store the settings file in the appropriate directory.
I'm not primarily a Windows user and am not very familiar with portable apps, so it would indeed be nice if somebody else could help with this task.
Ah, btw rather than hosting our own portable .zip package, I'd prefer a package following the guidelines of http://portableapps.com/ and for it to be uploaded there. That should help users managing their portable version of Tiled alongside other portable apps.