Having a hard time understanding git-fetch

Question

I am having a hard time understanding the nuances of git-fetch. I understand that doing a fetch, fetches the remote refs into a local tracking branch.

I

Answer 1

Note also that, with Git 2.5+ (Q2 2015), git merge FETCH_HEAD can merge multiple git fetch's.

See commit d45366e by Junio C Hamano (gitster), 26 Mar 2015.
^{(Merged by Junio C Hamano -- gitster -- in commit bcd1ecd, 19 May 2015)}

"git merge FETCH_HEAD" learned that the previous "git fetch" could be to create an Octopus merge, i.e. recording multiple branches that are not marked as "not-for-merge";
this allows us to lose an old style invocation "git merge HEAD $commits..." in the implementation of "git pull" script; the old style syntax can now be deprecated.

The git merge doc now mention:

When FETCH_HEAD (and no other commit) is specified, the branches recorded in the .git/FETCH_HEAD file by the previous invocation of git fetch for merging are merged to the current branch.

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Git 2.13 (Q2 2017) officially retires the old syntax for git merge.
See commit b439165 (26 Mar 2015) by Junio C Hamano (gitster).
^{(Merged by Junio C Hamano -- gitster -- in commit 1fdbfc4, 30 Mar 2017)}

merge: drop 'git merge HEAD ' syntax

Stop supporting "git merge HEAD " syntax that has been deprecated since October 2007, and issues a deprecation warning message since v2.5.0.

That means the warning message old-style "'git merge HEAD ' is deprecated." is no more.