archive: fix subst file generation

archive: fix subst file generation

Before the strbuf conversion, result was a char pointer.  The if
statement checked for it being not NULL, which meant that no
"$Format:...$" string had been found and no replacement had to be
made.  format_subst() returned NULL in that case -- the caller
then simply kept the original file content, as it was unaffected
by the expansion.

The length of the string being 0 is not the same as the string
being NULL (expansion to an empty string vs. no expansion at all),
so checking result.len != 0 is not a full replacement for the old
NULL check.

However, I doubt the subtle optimization explained above resulted
in a notable speed-up anyway.  Simplify the code and add the tail
of the file to the expanded string unconditionally.

[jc: added a test to expose the breakage this fixes]

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