有时git会建议git rm——cached来取消文件，有时git会重置HEAD文件。什么时候用哪个?

D:\code\gt2>git init
Initialized empty Git repository in D:/code/gt2/.git/
D:\code\gt2>touch a

D:\code\gt2>git status
# On branch master
#
# Initial commit
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       a
nothing added to commit but untracked files present (use "git add" to track)

D:\code\gt2>git add a

D:\code\gt2>git status
# On branch master
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#       new file:   a
#
D:\code\gt2>git commit -m a
[master (root-commit) c271e05] a
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 a

D:\code\gt2>touch b

D:\code\gt2>git status
# On branch master
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       b
nothing added to commit but untracked files present (use "git add" to track)

D:\code\gt2>git add b

D:\code\gt2>git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       new file:   b
#