[git-buildpackage] Re-creating gbp merge commit

[Otto Kekäläinen]

This is essentially a question of can git support rebasing or
cherry-picking merges, and unfortunately the answer is not. However,
you can cherry-pick multiple commits in a row, e.g.

With your current history:

± g-log twrightsman/master
* c8eb2d558a Limit ccache size to 3 GB 15 hours ago (twrightsman/master)
* 8a8a27f135 Pass ccache size limit into SCons build environment 15 hours ago
* f1950bae3d Release 3.6 and 4.3 to unstable 15 hours ago
* 441f17da57 Refresh copyright with latest upstream version 15 hours ago
* ecd5fc5ddb Update packaging for new upstream release 15 hours ago
*   d03b85fe1f Update upstream source from tag 'upstream/4.3+ds' 15 hours ago
|\
| * 1a9a036958 New upstream version 4.3+ds 15 hours ago
* | ef5fe3878e Update watch file to match latest upstream version 15 hours ago
* | 780bad55ed Add myself to d/copyright 2 days ago (twrightsman/debian/latest)
* | bbdfc6866c Refresh patches for new upstream version 2 days ago
(HEAD -> debian/latest, twrightsman/debian/3.x, origin/debian/latest)
* | f75a9019f7 Start changelog for new release 2 days ago
* | 12258d8a0c Update upstream source from tag 'upstream/3.6+ds' 2 days ago

You can do this:

git reset --hard ef5fe3878e
git cherry-pick f1950bae3d
git import-orig --uscan --verbose
git cherry-pick  ecd5fc5ddb 441f17da57 8a8a27f135 c8eb2d558a

..and you are back to where you started but f1950bae3d is not pre-merge