Just move the file as you would do without Git.
The commit window then looks ugly: a move is displayed as a delete/add
pair there.
But later on, when displaying the log, Git automatically detects moves
and renames - even when you did some small content changes at the same time.
And since 1.7.10 TortoiseGit supports following moves and renames when
displaying the log of a single file, via a checkbox.