shelve: store shelved node in a new data file
It is useful for a version of shelve not based on bundle and strip. Having
more data does not hurt the bundle based case so we introduce it in all cases
before doing more rework.
We also keep storing the patch using another method.
note: We could have this metadata file list all the file involved in the
shelve and use that for cleanup. This would be more future proof than having a
hard-coded list of file to purge. However, this is an adventure for another
series.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present