2箇所離れている場所を変更して
git add -pを利用すると変更箇所ごとに確認が出ます。

$ git add -p
diff --git a/test.txt b/test.txt
index 43eb1f4..63b004f 100644
--- a/test.txt
+++ b/test.txt
@@ -1,6 +1,7 @@
 test
 a
-2
+b
+30
 3
 4
 x
Stage this hunk [y,n,q,a,d,/,j,J,g,e,?]? y
@@ -36,4 +37,5 @@ z


 111
+99999
 212
Stage this hunk [y,n,q,a,d,/,K,g,e,?]? n

これで上の更新のみaddされます。

$ git diff
diff --git a/test.txt b/test.txt
index d10d14a..63b004f 100644
--- a/test.txt
+++ b/test.txt
@@ -37,4 +37,5 @@ z


 111
+99999
 212

下の更新はまだaddしていません。

add下部分のみの確認は

$ git diff --cached
diff --git a/test.txt b/test.txt
index 43eb1f4..d10d14a 100644
--- a/test.txt
+++ b/test.txt
@@ -1,6 +1,7 @@
 test
 a
-2
+b
+30
 3
 4
 x

* text.txtを変更 & test2.txtを作成を追加します。

$ git add -A
↑ git add . && git add -u と同じ意味です。

$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
#       modified:   test.txt
#       new file:   test2.txt
#

text.txtのadd をやめる。

$ git reset test.txt
Unstaged changes after reset:
M       test.txt

$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD ..." to unstage)
#
#       new file:   test2.txt
#
# Changes not staged for commit:
#   (use "git add ..." to update what will be committed)
#   (use "git checkout -- ..." to discard changes in working directory)
#
#       modified:   test.txt
#

git commit -v で diff –cached と commitを一気に。

$ git commit -v
add test2
# 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 ..." to unstage)
#
# new file:   test2.txt
#
# Changes not staged for commit:
#   (use "git add ..." to update what will be committed)
#   (use "git checkout -- ..." to discard changes in working directory)
#
# modified:   test.txt
#
diff --git a/test2.txt b/test2.txt
new file mode 100644
index 0000000..9daeafb
--- /dev/null
+++ b/test2.txt
@@ -0,0 +1 @@
+test

これで保存。

".git/COMMIT_EDITMSG" 22L, 597C written
[master 6e2f6c7] add test2
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 test2.txt