prepush: fix bug in warning message selection
In certain situations you would be told "you should pull and merge" even when
there is nothing to pull.
adding a
popping c.patch
popping b.patch
now at: a.patch
applying b.patch
applying c.patch
now at: c.patch
popping c.patch
now at: b.patch
% Using index
popping b.patch
now at: a.patch
applying b.patch
applying c.patch
now at: c.patch
% No warnings when using index
popping bug141421
popping bug314159
popping c.patch
now at: b.patch
applying c.patch
applying bug314159
now at: bug314159
% Detect ambiguous non-index
patch name "14" is ambiguous:
bug314159
bug141421
abort: patch 14 not in series