dpkg-divert: Return from a command function instead of directly exiting

This makes it possible to perform any necessary normal cleanup before
exiting, and unifies the usage with all other command functions.

debian/changelog

@@ -11,6 +11,8 @@ dpkg (1.18.4) UNRELEASED; urgency=low
   * Fix memory leaks in «dpkg --verify» and dpkg infodb format upgrade logic.
   * Merge all update-alternatives action handling into a single if-else-if
     block, to unify the code an allow a future switch into a shared library.
+  * Perform any necessary cleanups on normal exit from dpkg-divert --add and
+    --remove commands.
   * Build system:
     - Set PERL5LIB globally for the test suite to the local modules directory,
       to avoid using the system modules. Regression introduced in dpkg 1.17.8.

src/divertcmd.c

@@ -464,7 +464,7 @@ diversion_add(const char *const *argv)
 			if (opt_verbose > 0)
 				printf(_("Leaving '%s'\n"),
 				       diversion_describe(fnn_from->divert));
-			exit(0);
+			return 0;
 		}
 
 		ohshit(_("'%s' clashes with '%s'"),
@@ -595,7 +595,7 @@ diversion_remove(const char *const *argv)
 		if (opt_verbose > 0)
 			printf(_("Ignoring request to remove shared diversion '%s'.\n"),
 			       diversion_describe(contest));
-		exit(0);
+		return 0;
 	}
 
 	if (opt_verbose > 0)