clone: use better names for temp files
Before this commit, the file names are /tmp/tmpn8smvlr8
After this commit, they are more like /tmp/hg-clone-n8smvlr8/00manifest.ndb3qj52v6,
which makes it much clearer what these files correspond to.
Differential Revision: https://phab.mercurial-scm.org/D12623
[workspace]
members = ["hg-core", "hg-cpython", "rhg"]
exclude = ["chg", "hgcli"]