parsers: simplify error logic in compute_phases_map_sets
Since Py_XDECREF and free both accept NULL pointers, we can get by
with just two exit paths: one for success, and one for error.
This considerably simplifies reasoning about the possible ways to
exit from this function.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..