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

To synthesize the excellent original answers from @adardesign, @adswebwork and @AnkitVishwakarma, and comments from @Bdoserror, @Retsam, @seanf, and @torek, with additional documentation links and concise aliases...

Basic Commands

To reset a file that is assume-unchanged back to normal:

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

To list all files that are assume-unchanged:

git ls-files -v | grep '^[a-z]' | cut -c3-

To reset all assume-unchanged files back to normal:

git ls-files -v | grep '^[a-z]' | cut -c3- | xargs git update-index --no-assume-unchanged --

Note: This command which has been listed elsewhere does not appear to reset all assume-unchanged files any longer (I believe it used to and previously listed it as a solution):

git update-index --really-refresh

Shortcuts

To make these common tasks easy to execute in git, add/update the following alias section to .gitconfig for your user (e.g. ~/.gitconfig on a *nix or macOS system):

[alias]
    hide = update-index --assume-unchanged
    unhide = update-index --no-assume-unchanged
    unhide-all = ! git ls-files -v | grep '^[a-z]' | cut -c3- | xargs git unhide --
    hidden = ! git ls-files -v | grep '^[a-z]' | cut -c3-

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>