update watch file: use file format 4; track Gitlab source; mention gbp --uscan in usage comment; use substitution strings

debian/watch

@@ -1,4 +1,10 @@
-version=3
+version=4
+# check: uscan --report
+# update: gbp import-orig --upstream-vcs-tag=vX.Y.Z --uscan
 
-opts=dversionmangle=s/\+ds\d+$// \
-  https://downloads.syncevolution.org/syncevolution/sources/syncevolution-(.*).tar.gz 
+opts=\
+filenamemangle=s/.*?(@ANY_VERSION@@ARCHIVE_EXT@)/@PACKAGE@$1/,\
+uversionmangle=s/-(?=\d)/./g;s/-?pre/~pre/,\
+dversionmangle=auto \
+https://gitlab.freedesktop.org/SyncEvolution/syncevolution/-/tags \
+.*?@ANY_VERSION@@ARCHIVE_EXT@