Add Checkout Old Version Of A File as a git til.

git/checkout-old-version-of-a-file.md

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+# Checkout Old Version Of A File
+
+When you want to return to a past version of a file, you can reset to a past
+commit. When you don't want to abandon a bunch of other changes, this isn't
+going to cut it. Another option is to just checkout the particular file as
+it was at the time of a past commit.
+
+If the sha of that past commit is `72f2675` and the file's name is
+`some_file.rb`, then just use checkout like so:
+
+```
+$ git checkout 72f2675 some_file.rb
+```