git-debrebase: convert-from-gbp: Use same algorithm for finding upstream

The version is in the changelog, and thus upstream commitish can then
be found from as new-upstream.

This also means we now give the same error message, which is much
better than the one we had before.

Closes:#905062.

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

1 files changed, 10 insertions, 3 deletions

diff --git a/git-debrebase b/git-debrebase
index 5d3c742..2687ef1 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -1604,11 +1604,18 @@ sub cmd_make_patches () {
 }
 
 sub cmd_convert_from_gbp () {
-    badusage "needs 1 optional argument, the upstream git rev"
+    badusage "want only 1 optional argument, the upstream git commitish"
 	unless @ARGV<=1;
+
+    my $clogp = parsechangelog();
+    my $version = $clogp->{'Version'}
+	// die "missing Version from changelog";
+
     my ($upstream_spec) = @ARGV;
-    $upstream_spec //= 'refs/heads/upstream';
-    my $upstream = git_rev_parse $upstream_spec;
+
+    my ($upstream, $upstream_version) =
+	resolve_upstream_version($upstream_spec, $version);
+
     my $old_head = get_head();
 
     my $upsdiff = get_differs $upstream, $old_head;