patman: Allow creating patches for another branch

Add a -b option to allow patches to be created from a branch other than
the current one.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/main.py b/tools/patman/main.py
index 2432d31..0667541 100755
--- a/tools/patman/main.py
+++ b/tools/patman/main.py
@@ -31,6 +31,8 @@
 parser = OptionParser()
 parser.add_option('-H', '--full-help', action='store_true', dest='full_help',
        default=False, help='Display the README file')
+parser.add_option('-b', '--branch', type='str',
+                  help="Branch to process (by default, the current branch)")
 parser.add_option('-c', '--count', dest='count', type='int',
        default=-1, help='Automatically create patches from top n commits')
 parser.add_option('-i', '--ignore-errors', action='store_true',