bisect: fix traceback when command executable is not found

mercurial/commands.py

@@ -339,6 +339,8 b' def bisect(ui, repo, rev=None, extra=Non'
 
     if command:
         commandpath = util.find_exe(command)
+        if commandpath is None:
+            raise util.Abort(_("cannot find executable: %s") % command)
         changesets = 1
         try:
             while changesets:

tests/test-bisect

@@ -82,3 +82,7 b' hg bisect -s'
 hg bisect -s
 hg bisect -s 
 hg bisect -g
+
+echo % test invalid command
+hg bisect -r
+hg bisect --command 'foobar'

tests/test-bisect.out

@@ -303,3 +303,5 b' user: test'
 date:        Thu Jan 01 00:00:06 1970 +0000
 summary:     msg 6
 
+% test invalid command
+abort: cannot find executable: foobar