doc: Explain how to fetch commits directly

Author: MarcoFalke

doc/productivity.md

@@ -19,6 +19,7 @@ Table of Contents
    * [More conflict context with `merge.conflictstyle diff3`](#more-conflict-context-with-mergeconflictstyle-diff3)
 * [Reviewing code](#reviewing-code)
    * [Reduce mental load with `git diff` options](#reduce-mental-load-with-git-diff-options)
+   * [Fetch commits directly](#fetch-commits-directly)
    * [Reference PRs easily with `refspec`s](#reference-prs-easily-with-refspecs)
    * [Diff the diffs with `git range-diff`](#diff-the-diffs-with-git-range-diff)
 
@@ -164,9 +165,17 @@ When reviewing patches that change symbol names in many places, use `git diff --
 
 When reviewing patches that move code around, try using `git diff --patience commit~:old/file.cpp commit:new/file/name.cpp`, and ignoring everything except the moved body of code which should show up as neither `+` or `-` lines. In case it was not a pure move, this may even work when combined with the `-w` or `--word-diff` options described above. `--color-moved=dimmed-zebra` will also dim the coloring of moved hunks in the diff on compatible terminals.
 
+### Fetch commits directly
+
+Before inspecting any remotely created commit locally, it has to be fetched.
+This is possible via `git fetch origin <full_commit_hash>`. Even commits not
+part of any branch or tag can be fetched as long as the remote has not garbage
+collected them.
+
+
 ### Reference PRs easily with `refspec`s
 
-When looking at other's pull requests, it may make sense to add the following section to your `.git/config` file:
+As an alternative to fetching commits directly, when looking at pull requests by others, it may make sense to add the following section to your `.git/config` file:
 
 ```
 [remote "upstream-pull"]