Undo git update-index --assume-unchanged <file>

To get undo/show dir's/files that are set to assume-unchanged run this:

git update-index --no-assume-unchanged <file>

To get a list of dir's/files that are assume-unchanged run this:

git ls-files -v|grep '^h'

If this is a command that you use often - you may want to consider having an alias for it as well. Add to your global .gitconfig:

[alias]    hide = update-index --assume-unchanged    unhide = update-index --no-assume-unchanged

How to set an alias (if you don't know already):

git config --configLocation alias.aliasName 'command --options'

Example:

git config --global alias.hide 'update-index --assume-unchanged'git config... etc

After saving this to your .gitconfig, you can run a cleaner command.

git hide myfile.ext

or

git unhide myfile.ext

This git documentation was very helpful.

As per the comments, this is also a helpful alias to find out what files are currently being hidden:

[alias]    hidden = ! git ls-files -v | grep '^h' | cut -c3-

git update-index function has several option you can find typing as below:

git update-index --help

Here you will find various option - how to handle with the function update-index.

[if you don't know the file name]

git update-index --really-refresh 

[if you know the file name ]

git update-index --no-assume-unchanged <file>

will revert all the files those have been added in ignore list through.

git update-index --assume-unchanged <file>