pycompat: introduce identity function as a compat stub
I was sometimes too lazy to use 'str' instead of 'lambda a: a'. Let's add
a named function for that purpose.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state