patman: check git format.subjectprefix setting when generate patches prefix

For the local project, we may specified format.subjectprefix setting.
Then the patch will be formated as [Project_prefix][PATCH].
But patman will not check this setting. It will remove the
format.subjectprefix.

So This patch will let patman check this setting and add it as a
project prefix.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/gitutil.py b/tools/patman/gitutil.py
index 4c2c35b..9e739d8 100644
--- a/tools/patman/gitutil.py
+++ b/tools/patman/gitutil.py
@@ -545,6 +545,17 @@
     uemail = command.OutputOneLine('git', 'config', '--global', 'user.email')
     return uemail
 
+def GetDefaultSubjectPrefix():
+    """Gets the format.subjectprefix from local .git/config file.
+
+    Returns:
+        Subject prefix found in local .git/config file, or None if none
+    """
+    sub_prefix = command.OutputOneLine('git', 'config', 'format.subjectprefix',
+                 raise_on_error=False)
+
+    return sub_prefix
+
 def Setup():
     """Set up git utils, by reading the alias files."""
     # Check for a git alias file also