[StGIT PATCH] Better diagnostic for wrong branch configuration.

[Yann Dirson <ydirson@altern.org>]

If the branch.*.merge parameter does not name a valid remote head,
stgit would not rebase after a fetch, and would write instead
'Rebasing to "None" ... done'.

This patch makes this situation an error and tells the user what
to fix in his repo configuration.
---

 stgit/commands/pull.py |    9 ++++++++-
 stgit/git.py           |    5 ++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/stgit/commands/pull.py b/stgit/commands/pull.py
index 052ea2b..070db99 100644
--- a/stgit/commands/pull.py
+++ b/stgit/commands/pull.py
@@ -90,7 +90,14 @@ def func(parser, options, args):
     elif policy == 'fetch-rebase':
         out.info('Fetching from "%s"' % repository)
         git.fetch(repository)
-        rebase(git.fetch_head())
+        try:
+            target = git.fetch_head()
+        except git.GitException:
+            out.error('Could not find the remote head to rebase onto, pushing any patches back...')
+            post_rebase(applied, False, False)
+            raise CmdException, 'Could not find the remote head to rebase onto - fix branch.%s.merge in .git/config' % crt_series.get_name()
+
+        rebase(target)
     elif policy == 'rebase':
         rebase(crt_series.get_parent_branch())
 
diff --git a/stgit/git.py b/stgit/git.py
index 9e0f771..a0493bc 100644
--- a/stgit/git.py
+++ b/stgit/git.py
@@ -1003,11 +1003,14 @@ def fetch_head():
         m = re.match('^([^\t]*)\t\t', line)
         if m:
             if fetch_head:
-                raise GitException, "StGit does not support multiple FETCH_HEAD"
+                raise GitException, 'StGit does not support multiple FETCH_HEAD'
             else:
                 fetch_head=m.group(1)
     stream.close()
 
+    if not fetch_head:
+        raise GitException, 'No for-merge remote head found in FETCH_HEAD'
+
     # here we are sure to have a single fetch_head
     return fetch_head