t0021-conversion.sh: fix NoTerminatingSymbolAtEOF test

t0021-conversion.sh: fix NoTerminatingSymbolAtEOF test

The last line of the test file "expanded-keywords" ended in a newline,
which is a valid terminator for ident.  Use printf instead of echo to omit
it and thus really test if a file that ends unexpectedly in the middle of
an ident tag is handled properly.

Also take the oppertunity to calculate the expected ID dynamically
instead of hardcoding it into the test script.  This should make future
changes easier.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>