util: add a doctest for empty sha() calls

largefiles: replace tempfile.NamedTemporaryFile with tempfile.mkstemp This is consistent with the rest of Mercurial's code, mirroring the try-finally-unlink structure elsewhere. Furthermore, it fixes the case where largefiles throws an IOError on Windows when the temporary file is opened a second time by copytocacheabsolute. This patch creates the temporary file in the repo's largefiles store rather than /tmp, which might be a different filesystem.

util: allow sha1() with no args Normally this works because we replace util.sha1 with hashlib.sha1 after first use, but if the first user doesn't provide an arg, it breaks.