bisect: extract the 'reset' logic into its own function

This is part of small clean up movement. The bisect module seem more appropriate
to host the bisect logic. The cleanup itself is motivated by some higher level
cleanup around vfs and locking.

mercurial/commands.py

@@ -902,8 +902,7 b' def bisect(ui, repo, rev=None, extra=Non'
     cmdutil.checkunfinished(repo)
 
     if reset:
-        if repo.vfs.exists("bisect.state"):
-            repo.vfs.unlink("bisect.state")
+        hbisect.resetstate(repo)
         return
 
     state = hbisect.load_state(repo)

mercurial/hbisect.py

@@ -159,6 +159,11 b' def save_state(repo, state):'
                 f.write("%s %s\n" % (kind, hex(node)))
         f.close()
 
+def resetstate(repo):
+    """remove any bisect state from the repository"""
+    if repo.vfs.exists("bisect.state"):
+        repo.vfs.unlink("bisect.state")
+
 def get(repo, status):
     """
     Return a list of revision(s) that match the given status: