add -i, send-email, svn, p4, etc: use "git var GIT_EDITOR"

Use the new "git var GIT_EDITOR" feature to decide what editor to
use, instead of duplicating its logic elsewhere.  This should make
the behavior of commands in edge cases (e.g., editor names with
spaces) a little more consistent.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/fast-import/git-p4

@@ -729,13 +729,10 @@ class P4Submit(Command):
             tmpFile.write(submitTemplate + separatorLine + diff + newdiff)
             tmpFile.close()
             mtime = os.stat(fileName).st_mtime
-            defaultEditor = "vi"
-            if platform.system() == "Windows":
-                defaultEditor = "notepad"
             if os.environ.has_key("P4EDITOR"):
                 editor = os.environ.get("P4EDITOR")
             else:
-                editor = os.environ.get("EDITOR", defaultEditor);
+                editor = read_pipe("git var GIT_EDITOR")
             system(editor + " " + fileName)
 
             response = "y"