git p4: avoid shell when invoking git rev-list

Invoke git rev-list directly, avoiding the shell, in
P4Submit and P4Sync.  The overhead of starting extra
processes is significant in cygwin; this speeds things
up on that platform.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -1606,7 +1606,7 @@ def run(self, args):
         self.check()
 
         commits = []
-        for line in read_pipe_lines("git rev-list --no-merges %s..%s" % (self.origin, self.master)):
+        for line in read_pipe_lines(["git", "rev-list", "--no-merges", "%s..%s" % (self.origin, self.master)]):
             commits.append(line.strip())
         commits.reverse()
 
@@ -2644,7 +2644,8 @@ def importNewBranch(self, branch, maxChange):
 
     def searchParent(self, parent, branch, target):
         parentFound = False
-        for blob in read_pipe_lines(["git", "rev-list", "--reverse", "--no-merges", parent]):
+        for blob in read_pipe_lines(["git", "rev-list", "--reverse",
+                                     "--no-merges", parent]):
             blob = blob.strip()
             if len(read_pipe(["git", "diff-tree", blob, target])) == 0:
                 parentFound = True