test-convert-git: Use ~ instead of ^
Solaris sh interprets ^ as pipe symbol and the test thus failed in strange
ways. git can however use of HEAD~ instead of HEAD^.
adding foo
% no signatures
% hg sign 0
Signing 0:e63c23eaa88a
% hg sigs
hgtest 0:e63c23eaa88ae77967edcf4ea194d31167c478b0
% hg sigcheck 0
e63c23eaa88a is signed by:
hgtest