Method stating which commit is being played during an halted rebase

This will be useful to me at least. This way, I know that I can tell
my script to omit some specific commits. If you accept to merge it, I
may also do similar method for merges and cherry pick.

Author: Arthur-Milchior

Diff for: AUTHORS

@@ -33,5 +33,6 @@ Contributors are:
 -Steven Whitman <ninloot _at_ gmail.com>
 -Stefan Stancu <stefan.stancu _at_ gmail.com>
 -César Izurieta <cesar _at_ caih.org>
+-Arthur Milchior <arthur _at_ milchior.fr>
 
 Portions derived from other open source works and are clearly marked.

Diff for: git/repo/base.py

@@ -1062,3 +1062,14 @@ def has_separate_working_tree(self):
 
     def __repr__(self):
         return '<git.Repo "%s">' % self.git_dir
+
+    def currentlyRebasingOn(self):
+        """
+        :return: The hash of the commit which is currently being replayed while rebasing.
+
+        None if we are not currently rebasing.
+        """
+        rebase_head_file = osp.join(self.git_dir, "REBASE_HEAD")
+        if not osp.isfile(rebase_head_file):
+            return None
+        return open(rebase_head_file, "rt").readline().strip()