doc: don't use mutable default arguments
It appears our mutable default arguments checker doesn't find
arguments not on the first line of a function definition :(
The arguments aren't used, so the default value is irrelevant.
I found this when blackening this script in a future commit.
Differential Revision: https://phab.mercurial-scm.org/D7445
$ 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 ..