$ cat readme.txt
Git is a distributed version control system.
Git is free software distributed under the GPL.
Git has a mutable index called stage.
Git tracks changes.

$ git add readme.txt
$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       modified:   readme.txt
#

$ cat readme.txt 
Git is a distributed version control system.
Git is free software distributed under the GPL.
Git has a mutable index called stage.
Git tracks changes of files.

$ git commit -m "git tracks changes"
[master d4f25b6] git tracks changes
 1 file changed, 1 insertion(+)

$ git status
# 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:   readme.txt
#
no changes added to commit (use "git add" and/or "git commit -a")

$ git diff HEAD -- readme.txt 
diff --git a/readme.txt b/readme.txt
index 76d770f..a9c5755 100644
--- a/readme.txt
+++ b/readme.txt
@@ -1,4 +1,4 @@
 Git is a distributed version control system.
 Git is free software distributed under the GPL.
 Git has a mutable index called stage.
-Git tracks changes.
+Git tracks changes of files.