doc: replace gfd() function to recommendation to use git diff-range

CONTRIBUTING.md

@@ -348,20 +348,9 @@ a worthwhile change based on the judgement of the maintainers.
 
 When someone rebases their PR, it can often be very difficult to ensure that
 extra changes were not included in that force push. This changes could be anything
-from merge conflicts to someone attempting to sneak something into the PR. To check
-that a PR is the same before and after force push, you can use the following function.
-Place this function in your `~/.bashrc`. In order for this function to work, both the
-before and after commits must be present locally.
-
-```
-function gfd() {
-        local fp1=$(git show-branch --merge-base develop $1)
-        local fp2=$(git show-branch --merge-base develop $2)
-        echo fp1=$fp1
-        echo fp2=$fp2
-        diff --color=always -u -I'^[^-+]' <(git diff $fp1..$1) <(git diff $fp2..$2)
-}
-```
+from merge conflicts to someone attempting to sneak something into the PR.
+In order for `git range-diff` to work, both the before and after commits must be
+present locally. See chapter [rebasing changes](#rebasing-changes)
 
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