doc: patch-id: add script example

The utility and usability of git-patch-id(1) was discussed
relatively recently:[1]

    Using "git patch-id" is definitely in the "write a script for it"
    category. I don't think I've ever used it as-is from the command
    line as part of a one-liner. It's very much a command that is
    designed purely for scripting, the interface is just odd and baroque
    and doesn't really make sense for one-liners.

    The typical use of patch-id is to generate two *lists* of patch-ids,
    then sort them and use the patch-id as a key to find commits that
    look the same.

The command doc *could* use an example, and since it is a mapper command
it makes sense for that example to be a little script.

Mapping the commits of some branch to an upstream ref allows us to
demonstrate generating two lists, sorting them, joining them, and
finally discarding the patch ID lookup column with cut(1).

† 1: https://lore.kernel.org/workflows/CAHk-=wiN+8EUoik4UeAJ-HPSU7hczQP+8+_uP3vtAy_=YfJ9PQ@mail.gmail.com/

Inspired-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-patch-id.adoc

@@ -68,6 +68,46 @@ This is the default if `patchid.stable` is set to `true`.
 +
 This is the default.
 
+EXAMPLES
+--------
+
+linkgit:git-cherry[1] shows what commits from a branch have patch ID
+equivalent commits in some upstream branch. But it only tells you
+whether such a commit exists or not. What if you wanted to know the
+relevant commits in the upstream? We can use this command to make a
+mapping between your branch and the upstream branch:
+
+----
+#!/bin/sh
+
+upstream="$1"
+branch="$2"
+test -z "$branch" && branch=HEAD
+limit="$3"
+if test -n "$limit"
+then
+    tail_opts="$limit".."$upstream"
+else
+    since=$(git log --format=%aI "$upstream".."$branch" | tail -1)
+    tail_opts=--since="$since"' '"$upstream"
+fi
+for_branch=$(mktemp)
+for_upstream=$(mktemp)
+
+git rev-list --no-merges "$upstream".."$branch" |
+    git diff-tree --patch --stdin |
+    git patch-id  --stable | sort >"$for_branch"
+git rev-list --no-merges $tail_opts |
+    git diff-tree --patch --stdin |
+    git patch-id  --stable | sort >"$for_upstream"
+join -a1 "$for_branch" "$for_upstream" | cut -d' ' -f2,3
+rm "$for_branch"
+rm "$for_upstream"
+----
+
+Now the first column shows the commit from your branch and the second
+column shows the patch ID equivalent commit, if it exists.
+
 GIT
 ---
 Part of the linkgit:git[1] suite