git fetch vs. git fetch origin master have different effects on tracking branch

Question

This is mostly of the nature of a curiosity as I'm trying to get familiar with Git. I have looked at the documentation for 'git fetch' but I don't see an obvious explanation for the below. Thanks in advance, and apologies if this is howlingly obvious.

1) From a central repository, say GitHub, I clone a repository named website on each of two machines, HostA and HostB.

2) on HostA, I make a change to a file, say README.txt, and commit it.
At this point on HostA, the commits for branches master and origin/master are, as expected different since I haven't pushed yet

git show master
git show origin/master

report different hashes (since master has the change and origin/master does not)

3) Once I push, they are after that the same.

---

4) Now, over on HostB, if I do the following:

git fetch
git merge FETCH_HEAD

afterwards, on HostB master and origin/master report the same hash when queried with git show

BUT

if instead I had done, on HostB:

git fetch origin master
git merge FETCH_HEAD

at that point the hashes still differ.

git show origin
git show origin/master

report different hashes

The tracking branch origin/master isn't updated until I do a plain git fetch

Why is this?

Answer 1

If your branch has an associated remote tracking branch that means its configuration is like:

git config branch.[branch-name].remote [remote-name]
git config branch.[branch-name].merge [remote-master]

The key part of git fetch which explain the difference between the two commands is:

<refspec>

The format of a <refspec> parameter is an optional plus +, followed by the source ref <src>, followed by a colon :, followed by the destination ref <dst>.
The remote ref that matches <src> is fetched, and if <dst> is not empty string, the local ref that matches it is fast-forwarded using <src>.

Let me repeat it:

if <dst> is not empty string, the local ref that matches it is fast-forwarded using <src>.
Knowing that:

• git fetch is equivalent to git fetch origin master:master (from the default value of your branch config), so it will update the remote tracking branch: the destination of the refspec is specified for you.

• git fetch origin master is equivalent to "git fetch origin master:", not to "git fetch origin master:master"; it stores fetched value of 'master' branch (of remote 'origin') in FETCH_HEAD, and not in 'master' branch or remote-tracking 'remotes/origin/master' branch (from Jakub Narębski's answer)
  In other words, you didn't specify the destination of your refspec