Add Previous Visual Selection as a vim til.

vim/previous-visual-selection.md

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+# Previous Visual Selection
+
+Typing `gv` in normal mode will re-select the previous visual selection.
+This makes it easy to re-select a specific block of text. For instance, if
+you are performing a search and replace on a visual selection and you didn't
+get the regex quite, you can quickly type `gv` and then edit the regex of
+your previous command.