bundle2.unbundlepart: implement seek()
This implements a seek() method for unbundlepart. This allows on-disk bundle2
parts to behave enough like files for bundlerepo to handle them. A future
patch will add support for bundlerepo to read the bundle2 files that are
written when the experimental.strip-bundle2-version config option is used.
#require execbit
$ hg init
$ echo a > a
$ hg ci -Am'not executable'
adding a
$ chmod +x a
$ hg ci -m'executable'
$ hg id
79abf14474dc tip
Make sure we notice the change of mode if the cached size == -1:
$ hg rm a
$ hg revert -r 0 a
$ hg debugstate
n 0 -1 unset a
$ hg status
M a
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg id
d69afc33ff8a
$ test -x a && echo executable -- bad || echo not executable -- good
not executable -- good