archive: defer opening the output until a file is matched...
archive: defer opening the output until a file is matched
Before, if no file is matched, an error is thrown, but the archive is
created anyway. When using hgweb, an error 500 is returned as the
response body already exists when the error is seen.
Afterwards, the archive is created before the first match is emitted.
If no match is found, no archive is created. This is more consistent
behavior as an empty archive is not a representable in all output
formats, e.g. tar archives.
