create-pull-request: Keep commit hash to be pulled in cover email

The cover email mangling in create-pull-request was cutting off the
actual commit hash to be pulled, making it difficult to verify that the
changes a maintainer merges exactly match those intended by the pull
request author.

The extra lines we want to include are, for example from a recent
whinlatter stable branch PR:

    for you to fetch changes up to 6c4c6d39ea3202d756acc13f8ce81b114a468541:

      cups: upgrade from 2.4.14 to 2.4.15 (2025-12-29 09:49:31 -0800)

Signed-off-by: Paul Barker <paul@pbarker.dev>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

Author: pbrkr

scripts/create-pull-request

@@ -219,7 +219,7 @@ fi
 
 # The cover letter already has a diffstat, remove it from the pull-msg
 # before inserting it.
-sed -n "0,\#$REMOTE_URL# p" "$PM" | sed -i "/BLURB HERE/ r /dev/stdin" "$CL"
+sed -n "0,\#^----------------------------------------------------------------# p" "$PM" | sed -i "/BLURB HERE/ r /dev/stdin" "$CL"
 rm "$PM"
 
 # If this is an RFC, make that clear in the cover letter