git-debrebase: Snag on repeated conversions

If debrebase-last says we are ahead of some previous gdr work, it is
carrying on is unlikely to be helpful.  It would only be sensible if
the package is vacillating between branch formats, and this is
reflected and visible in the branch history.

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

1 files changed, 3 insertions, 0 deletions

diff --git a/git-debrebase b/git-debrebase
index 059d02b..d80c8d3 100755
--- a/git-debrebase
+++ b/git-debrebase
@@ -2416,6 +2416,9 @@ sub begin_convert_from () {
 
     fail "ffq-prev exists, this is already managed by git-debrebase!"
 	if $ffq_prev && git_get_ref $ffq_prev;
+    snag 'already-converted',
+	"ahead of debrebase-last, this is already managed by git-debrebase!"
+	if $gdrlast && git_get_ref $gdrlast && is_fast_fwd $gdrlast, $head;
     return ($head, undef);
 }