op/magic.t - make $SIG{ALRM} local test deal with inherited IGNORE'd signal

Signal ignores can be set up by a parent process and the child process
will inherit them. So in some situations when we test $SIG{ALRM} might
be "IGNORE" and not undef. So instead of just expecting undef, check
what it was before the localization, and then check it is still that
value afterwards.

I found this while running make test via rebase --exec, something like
this reduction (from Matthew Horsfall):

    $ git rebase --exec='perl -le"print \$SIG{ALRM}"' HEAD~1
    IGNORE

A similar case for a different signal would be this example (from Leon
Timmermans):

    $ nohup perl -E 'say $SIG{HUP}' 2>/dev/null | cat
    IGNORE

1 files changed, 5 insertions, 1 deletions

diff --git a/t/op/magic.t b/t/op/magic.t
index 442735df49..d879406f58 100644
--- a/t/op/magic.t
+++ b/t/op/magic.t
@@ -932,10 +932,14 @@ SKIP: {
     }
 }
 
+# in some situations $SIG{ALRM} might be 'IGNORE', eg:
+# git rebase --exec='perl -e "print \$SIG{ALRM}" && git co -f' HEAD~2
+# will print out 'IGNORE'
+my $sig_alarm_expect= $SIG{ALRM};
 {
 	local %SIG = (%SIG, ALRM => sub {})
 };
-is $SIG{ALRM}, undef;
+is $SIG{ALRM}, $sig_alarm_expect, '$SIG{ALRM} is as expected';
 
 # test case-insignificance of %ENV (these tests must be enabled only
 # when perl is compiled with -DENV_IS_CASELESS)