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 symlink
https://bz.mercurial-scm.org/1438
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch