dpkg-divert: Only check for --divert being absolute when setting it

There's no point in checking it at --add time because even if it was
not specified and it got constructed from filename, it can never be
non-absolute as filename has already been checked for that.

src/divertcmd.c

@@ -409,8 +409,6 @@ diversion_add(const char *const *argv)
 		m_asprintf(&str, "%s.distrib", filename);
 		opt_divertto = str;
 	}
-	if (opt_divertto[0] != '/')
-		badusage(_("filename \"%s\" is not absolute"), opt_divertto);
 
 	if (strcmp(filename, opt_divertto) == 0)
 		badusage(_("cannot divert file '%s' to itself"), filename);
@@ -686,6 +684,8 @@ setdivertto(const struct cmdinfo *cip, const char *value)
 {
 	opt_divertto = value;
 
+	if (opt_divertto[0] != '/')
+		badusage(_("filename \"%s\" is not absolute"), opt_divertto);
 	if (strchr(opt_divertto, '\n') != NULL)
 		badusage(_("divert-to may not contain newlines"));
 }

src/t/100_dpkg_divert.t

@@ -33,7 +33,7 @@ if (! -x "@dd") {
     exit(0);
 }
 
-plan tests => 235;
+plan tests => 237;
 
 sub cleanup {
     system("rm -rf $tmpdir && mkdir -p $testdir");
@@ -138,7 +138,8 @@ call_divert(['--version'], expect_stdout_like => qr/.*dpkg-divert.*free software
 call_divert_badusage(['--jachsmitbju'], qr/unknown option/);
 call_divert_badusage(['--add', '--remove'], qr/(conflicting|two).*remove.*add.*/s);
 call_divert_badusage(['--divert'], qr/(takes a value|needs.*argument)/);
-call_divert_badusage(['--divert', "foo\nbar"], qr/newline/);
+call_divert_badusage(['--divert', "foo"], qr/absolute/);
+call_divert_badusage(['--divert', "/foo\nbar"], qr/newline/);
 call_divert_badusage(['--package'], qr/(takes a value|needs.*argument)/);
 call_divert_badusage(['--package', "foo\nbar"], qr/newline/);