packaging: extract pre-computed version when running from plain tarball
The tarballs created by setup.py are not including the usual archive
markers that `hg archive` leaves, so the rewrite of the version number
computation actually left the empty version string around. This meant
that installations from PyPI would use 0.0.0 as version string.
Differential Revision: https://phab.mercurial-scm.org/D10502
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev linkrev nodeid p1 p2
0 0 b80de5d13875 000000000000 000000000000
1 1 0376abec49b8 000000000000 000000000000
$ cd ..