git-debrebase: merge: Refactor interchange merge-base computation

We need to unpack this a bit so that we can insert some wrecknote
stuff, which will come in a moment.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

1 files changed, 13 insertions, 3 deletions

diff --git a/git-debrebase b/git-debrebase
index 1e8de52..f93adef 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -1235,12 +1235,22 @@ sub walk ($;$$$) {
 	    # breakwater merge base as an ancestor.
 
 	    my @ibs =
-		grep { is_fast_fwd $bwb, $_ }
 		grep /./,
 		split /\n/,
 		cmdoutput @ibcmd;
-	    my ($ib) = @ibs
-		or $nomerge->("no suitable interchange merge base");
+
+            @ibs or confess 'internal error, expected anchor at least ?';
+
+	    my $ib;
+	    foreach my $tibix (0..$#ibs) {
+		my $tib = $ibs[$tibix];
+		my $ff = is_fast_fwd $bwb, $tib;
+		next unless $ff;
+		next if $ib;
+		$ib = $tib;
+	    }
+
+	    $ib or $nomerge->("no suitable interchange merge base");
 
 	    $prline->("  VanillaMerge Base");
 	    $prprdelim->();