From the textMate manual:
4.6 Moving Text
4.6.1 Increase/Decrease Indent Level
In the Text menu there is a Shift Left and Shift Right action bound to ⌘[ and ⌘]. These will increase and decrease the indent by the size of one tab.
On many european key layouts these keys are rather awkward, so in addition to these, you can also use ⌥⇥ and ⌥⇤ (where ⇤ is achieved using ⇧⇥).
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http://manual.macromates.com/en/working_with_text#re-indent_text.html
