fix: refactor getrevstofix() to define revisions first, then validate them

This refactoring makes it easier to add a new way of specifying
revisions (I'm about to add a `--source`, which adds the specified
revisions and their descendants).

Differential Revision: https://phab.mercurial-scm.org/D8285

hgext/fix.py

@@ -400,15 +400,16 def getworkqueue(ui, repo, pats, opts, r
 def getrevstofix(ui, repo, opts):
     """Returns the set of revision numbers that should be fixed"""
     revs = set(scmutil.revrange(repo, opts[b'rev']))
+    if opts.get(b'working_dir'):
+        revs.add(wdirrev)
     for rev in revs:
         checkfixablectx(ui, repo, repo[rev])
-    if revs:
+    # Allow fixing only wdir() even if there's an unfinished operation
+    if not (len(revs) == 1 and wdirrev in revs):
         cmdutil.checkunfinished(repo)
         rewriteutil.precheck(repo, revs, b'fix')
-    if opts.get(b'working_dir'):
-        revs.add(wdirrev)
-        if list(merge.mergestate.read(repo).unresolved()):
-            raise error.Abort(b'unresolved conflicts', hint=b"use 'hg resolve'")
+    if wdirrev in revs and list(merge.mergestate.read(repo).unresolved()):
+        raise error.Abort(b'unresolved conflicts', hint=b"use 'hg resolve'")
     if not revs:
         raise error.Abort(
             b'no changesets specified', hint=b'use --rev or --working-dir'