git-p4: compare to singletons with "is" and "is not"

PEP8 recommends that comparisons with singletons such as None should be
done with "is" and "is not", and never equality operators.

This guideline is described here:
https://www.python.org/dev/peps/pep-0008/#programming-recommendations

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -857,7 +857,7 @@ def p4Where(depotPath):
             if data[:space] == depotPath:
                 output = entry
                 break
-    if output == None:
+    if output is None:
         return ""
     if output["code"] == "error":
         return ""
@@ -879,7 +879,7 @@ def currentGitBranch():
 
 
 def isValidGitDir(path):
-    return git_dir(path) != None
+    return git_dir(path) is not None
 
 
 def parseRevision(ref):
@@ -4425,7 +4425,7 @@ def main():
     global verbose
     verbose = cmd.verbose
     if cmd.needsGit:
-        if cmd.gitdir == None:
+        if cmd.gitdir is None:
             cmd.gitdir = os.path.abspath(".git")
             if not isValidGitDir(cmd.gitdir):
                 # "rev-parse --git-dir" without arguments will try $PWD/.git