Do not fail if SKIP_DCH is unset; use 'true' for checking for SKIP_DCH; inform user about it

The script runs under 'set -u' so we have to use "${SKIP_DCH:-}"
to make sure the script doesn't fail if SKIP_DCH is *not* set
(being the default usually).

We already check for 'true' in different settings, so let's keep
it homogeneous and use 'true' instead of '1'.

Also inform the user about when SKIP_DCH is being used to make
debugging easier.

Related to previous commit (201ec1e378).

scripts/generate-git-snapshot

@@ -166,31 +166,33 @@ else
 fi
 
 # Allow skipping of changelog creation, e.g. to build a branch as-is
-if [ "$SKIP_DCH" != "1" ]; then
-    if [ -n "${tag:-}" ] ; then
-      echo "Tag parameter found, not touching the changelog."
-    elif [ "${1:-}" = "identify" ] ; then
-      echo "Trying to identify latest tag / merge..."
-
-      if ! git describe $(git rev-list --tags='[^u]*' --max-count=1 HEAD) >/dev/null ; then
-        echo "Failed to identify latest change, falling back to auto mode."
-        git_dch_auto
-      else
-        identify_latest_change
-      fi
-    elif [ -r debian/gbp.conf ] ; then
-      echo "Found debian/gbp.conf, using git-dch with auto mode."
+if [ "${SKIP_DCH:-}" = "true" ] ; then
+  echo "*** SKIP_DCH is set, not modifying debian/changelog therefore ***"
+else
+  if [ -n "${tag:-}" ] ; then
+    echo "Tag parameter found, not touching the changelog."
+  elif [ "${1:-}" = "identify" ] ; then
+    echo "Trying to identify latest tag / merge..."
+
+    if ! git describe $(git rev-list --tags='[^u]*' --max-count=1 HEAD) >/dev/null ; then
+      echo "Failed to identify latest change, falling back to auto mode."
       git_dch_auto
     else
-      echo "Using git-dch with auto mode."
-      git_dch_auto
+      identify_latest_change
     fi
+  elif [ -r debian/gbp.conf ] ; then
+    echo "Found debian/gbp.conf, using git-dch with auto mode."
+    git_dch_auto
+  else
+    echo "Using git-dch with auto mode."
+    git_dch_auto
+  fi
 
-    # get rid of "UNRELEASED" distribution header, only
-    # necessary if the debian/changelog was modified
-    if [ -z "${tag:-}" ] ; then
-      debchange --release ""
-    fi
+  # get rid of "UNRELEASED" distribution header, only
+  # necessary if the debian/changelog was modified
+  if [ -z "${tag:-}" ] ; then
+    debchange --release ""
+  fi
 fi
 
 # prepare orig.tar.gz using pristine-tar, but without actually building the source package