为什么指定分支的 git fetch 与未指定分支的 fetch 不匹配(出于不同的目的)

Question

如果我做 git fetch origin master do git diff ...如果我做 git fetch（不指定分支）然后做 git diff ，则 origin 没有相同的结果......起源（见下文）。为什么？（请注意：git 版本 1.7.3.1.msysgit.0）

git init parent && cd parent
echo 'version1' > contents.txt
git add contents.txt
git commit -m "version1"
cd ..
git clone parent child
cd child
echo 'child' >> contents.txt
git commit -a -m "Child"
cd ../parent
echo 'parent' >> contents.txt
git commit -a -m "Parent"
cd ../child
git fetch origin master
git diff ...origin
echo Expected diff here and it wasn't there!
git fetch
git diff ...origin
echo Ok, diff appeared properly now!

Answer 1

git fetch origin master 不会将更新的提交指针存储在 .git/refs/remotes/origin/master 中，而是将其存储在 .git/FETCH_HEAD 中，如示例的输出所示：

$ git fetch origin master
remote: Counting objects: 5, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
From /tmp/parent
* branch            master     -> FETCH_HEAD

如果您想查看差异，只需发出git diff ...FETCH_HEAD：

$ git diff ...FETCH_HEAD
diff --git a/contents.txt b/contents.txt
index 5bdcfc1..1f428af 100644
--- a/contents.txt
+++ b/contents.txt
@@ -1 +1,2 @@
 version1
+parent

您可以在手册页中查看原因：

The ref names and their object names of fetched refs are stored in .git/FETCH_HEAD. 
This information is left for a later merge operation done by git merge.

另一方面，当您发出 git fetch origin 时，它会更新所有本地跟踪跟踪分支。从您的示例中：

$ git fetch origin
From /tmp/parent
e23f025..4ea3d15  master     -> origin/master

再次从手册页：

Update the remote-tracking branches:
       $ git fetch origin
   The above command copies all branches from the remote refs/heads/ namespace and stores them to the local refs/remotes/origin/ namespace, unless the
   branch.<name>.fetch option is used to specify a non-default refspec.