$ git init test
Initialized empty Git repository in test/.git/

$ cd test

$ echo hello > file

$ git add file

$ git commit -m "adding file"
[master (root-commit) 6f0412e] adding file
 1 file changed, 1 insertion(+)
 create mode 100644 file

$ git st
# On branch master
nothing to commit (working directory clean)

$ echo file > .gitignore

$ git st
# On branch master
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	.gitignore
nothing added to commit but untracked files present (use "git add" to track)

$ vim file # make some changes

$ git st
# On branch master
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#	modified:   file
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	.gitignore
no changes added to commit (use "git add" and/or "git commit -a")

# although 'file' has been added to .gitignore, changes show up because it was
# already tracked

$ git rm --cached file
rm 'file'

$ git st
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#	deleted:    file
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	.gitignore

$ git commit -m "deleted file from repo"
[master 0e9cd8f] deleted file from repo
 1 file changed, 1 deletion(-)
 delete mode 100644 file

$ vim file

$ git st
# On branch master
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	.gitignore
nothing added to commit but untracked files present (use "git add" to track)

$ vim .gitignore # now remove the line which says 'file'

$ git st
# On branch master
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	.gitignore
#	file
nothing added to commit but untracked files present (use "git add" to track)

# file shows up again as untracked, as I expected