How to see changes to a file before commit?

To see all the diff in tracked files but not staged:

git diff

or

git diff path/to/a/given/file

to see the diff only for a file. You can also see the diff in a given sub-directory of your project:

git diff path/to/a/dir/

If you have already staged the changes with git add, you can see what patch you have staged with

git diff --staged

You can also specify a path with --staged.

Make sure you've staged some changes. Otherwise, git commit -v will show you a block similar to what you posted, but not do anything. You can stage changes manually with git add, or if the files are already versioned, you can use git commit -a -v to stage and commit the changes.

For example:

$ echo "more foo" >> foo.txt
$ git commit -v
# 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:   foo.txt
#
no changes added to commit (use "git add" and/or "git commit -a")

Staging the change shows the diff with git commit -v:

:: git add foo.txt
:: GIT_EDITOR=cat git commit -v

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   modified:   foo.txt
#
diff --git a/foo.txt b/foo.txt
index 257cc56..a521556 100644
--- a/foo.txt
+++ b/foo.txt
@@ -1 +1,2 @@
 foo
+more foo
Aborting commit due to empty commit message.

If you just want to see the diff without committing, use git diff to see unstaged changes, git diff --cached to see changes staged for commit, or git diff HEAD to see both staged and unstaged changes in your working tree.

UPDATE: given your edit, what you really want are the git diff derivatives above. I'm not sure how Aptana Studio works. It may not follow the typical command line git flow. On the command line, you'd stage your changes, and then commit. And the above git diff commands are what you'd use to examine those changes. I typically alias them as git unstaged, git staged, and git both by adding this to my ~/.gitconfig:

[alias]
    # show difference between working tree and the index
    unstaged = diff

    # show difference between the HEAD and the index
    staged = diff --cached

    # show staged and unstaged changes (what would be committed with "git commit -a")
    both = diff HEAD