Add Watch This Run Repeatedly as a zsh til.

zsh/watch-this-run-repeatedly.md

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+# Watch This Run Repeatedly
+
+I usually reach for a quick bash for loop when I want to run a particular
+process a bunch of times in a row. The `watch` command is another way to
+run a process repeatedly.
+
+```
+watch rspec spec/some/test.rb
+```
+
+The default is 2 seconds in between subsequent executions of the command.
+The period can be changed with the `-n` flag though:
+
+```
+watch -n 2 rspec spec/some/test.rb
+```