Rework generate-git-snapshot to make sure quilt can't break the build

When using the dpkg 3.0 (quilt) format (see debian/source/format)
there might be quilt patches still applied in the source tree,
either because of a broken build or under certain - though not yet
clearly identified - situations which seem to end with a
"diff: standard output: Broken pipe" message in the build log.

This then might end up in a build failure like:

| + cd /var/lib/jenkins/workspace/glusterfs-source
| + dpkg-source -i -I -b source
| dpkg-source: info: using source format `3.0 (quilt)'
| diff: standard output: Broken pipe
| diff: standard output: Broken pipe
| dpkg-source: info: building glusterfs using existing ./glusterfs_3.2.7.orig.tar.gz
| dpkg-source: info: local changes detected, the modified files are:
|  source/ChangeLog
|  source/doc/gluster.8
|  source/doc/glusterfs.8
|  source/libglusterfs/src/common-utils.c
|  source/rpc/rpc-lib/src/rpcsvc.c
|  source/xlators/mgmt/glusterd/src/glusterd-rpc-ops.c
| dpkg-source: error: aborting due to unexpected upstream changes, see /tmp/glusterfs_3.2.7-2~bpo60+1.diff.3Wkisr
| dpkg-source: info: you can integrate the local changes with dpkg-source --commit

While everything has been cleanly integrated in debian/patches/
the build process fails because it looks like there have been
modifications to upstream sources whereas it's just wrong
information about patches applied.

So instead make sure we enter a clean git repository before doing
any changes (this can be skipped via SKIP_GIT_CLEANUP=true) and
also unapply all present quilt patches (this can be skipped via
SKIP_QUILT_CLEANUP=true). Finally replace the dpkg-source call
with dpkg-buildpackage and its --source-option=--unapply-patches
feature. While the combination of the git cleanup, quilt cleanup
and dpkg-source->dpkg-buildpackage is more than explicitely
necessary this provides more flexibility if needed.

Thanks: Raphael Hertzog <hertzog@debian.org> for his dpkg related feedback

debian/control

@@ -14,6 +14,7 @@ Architecture: all
 Depends: cowbuilder,
          devscripts,
          dpkg-dev,
+         quilt,
          reprepro,
          sudo | sudo-ldap,
          ${misc:Depends}

scripts/generate-git-snapshot

@@ -227,6 +227,30 @@ changelog_generation() {
   fi
 }
 
+git_cleanup() {
+  if [ -n "${SKIP_GIT_CLEANUP:-}" ] ; then
+    echo "*** Skipping git_cleanup as requested via SKIP_QUILT_CLEANUP ***"
+    return 0
+  fi
+
+  echo "*** Cleaning git repository to make sure there are no modified files ***"
+  echo "*** Note: You can skip this step by setting SKIP_GIT_CLEANUP=true ***"
+  git clean -fxd
+  git reset --hard HEAD
+}
+
+quilt_cleanup() {
+  if [ -n "${SKIP_QUILT_CLEANUP:-}" ] ; then
+    echo "*** Skipping quilt_cleanup as requested via SKIP_QUILT_CLEANUP ***"
+    return 0
+  fi
+
+  echo "*** Unapplying all possibly applied quilt patches and removing quilt directory ***"
+  echo "*** Note: You can skip this step by setting SKIP_QUILT_CLEANUP=true ***"
+  quilt pop -a || true
+  rm -rf .pc
+}
+
 # main execution
 echo "*** source package build phase ***"
 rm -f ./* || true
@@ -238,6 +262,8 @@ fi
 
 cd "$SOURCE_DIRECTORY"
 
+git_cleanup
+
 # make sure common branches are available for git-buildpackage
 create_local_branch upstream
 create_local_branch debian
@@ -264,9 +290,11 @@ else
   git-buildpackage ${GBP_OPTS}
 fi
 
+quilt_cleanup
+
 # build source package, run before switching back to previous branch
 # to get the actual requested version
-( cd "$WORKSPACE" ; dpkg-source -i -I -b "$SOURCE_DIRECTORY" )
+dpkg-buildpackage -uc -us -nc -d -S -i -I --source-option=--unapply-patches
 
 # revert to original debian/changelog to avoid merge conflicts
 git checkout -- $(readlink -f debian/changelog)