shelve: add an ability to write key-val data to a new type of shelve files

Obsolescense-based shelve only needs metadata stored in .hg/shelved
and if feels that this metadata should be stored in a
simplekeyvaluefile format for potential extensibility purposes.
I want to avoid storing it in an unstructured text file where
order of lines determines their semantical meanings (as now
happens in .hg/shelvedstate. .hg/rebasestate and I suspect other
state files as well).

Not included in this series, I have ~30 commits, doubling test-shelve.t
in size and testing almost every tested shelve usecase for obs-shelve.
Here's the series for the curious now: http://pastebin.com/tGJKx0vM
I would like to send it to the mailing list and get accepted as well,
but:
1. it's big, so should I send like 6 patches a time or so?
2. instead of having a commit per test case, it more like
   a commit per some amount of copy-pasted code. I tried to keep
   it meaningful and named commits somewhat properly, but it is
   far from this list standards IMO. Any advice on how to get it
   in without turning it into a 100 commits and spending many
   days writing descriptions?
3. it makes test-shelve.t run for twice as long (and it is already
   a slow test). Newest test-shelve.r runs for ~1 minute.

hgext/shelve.py

@@ -63,7 +63,7 b" testedwith = 'ships-with-hg-core'"
 
 backupdir = 'shelve-backup'
 shelvedir = 'shelved'
-shelvefileextensions = ['hg', 'patch']
+shelvefileextensions = ['hg', 'patch', 'oshelve']
 # universal extension is present in all types of shelves
 patchextension = 'patch'
 
@@ -154,6 +154,12 b' class shelvedfile(object):'
         bundle2.writebundle(self.ui, cg, self.fname, btype, self.vfs,
                                 compression=compression)
 
+    def writeobsshelveinfo(self, info):
+        scmutil.simplekeyvaluefile(self.vfs, self.fname).write(info)
+
+    def readobsshelveinfo(self):
+        return scmutil.simplekeyvaluefile(self.vfs, self.fname).read()
+
 class shelvedstate(object):
     """Handle persistence during unshelving operations.