[StGit PATCH 1/2] Handle "git diff-tree --stdin" error

[Samuel Tardieu <sam@rfc1149.net>]

If, for any reason, one of the SHA1 is not a commit object,
"git diff-tree --stdin" will signal an error on its standard
error and will only issue the requested "end" pattern.

"stg diff" could wait forver for the right references to
happen. With this patch, we now get:

% stg diff
stg diff: error: Object 6daaab2215a3d277a315d6938bdda86b2f75f6af not a commit
---
 stgit/lib/git.py |    6 +++---
 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/stgit/lib/git.py b/stgit/lib/git.py
index 4f210f8..45c0b32 100644
--- a/stgit/lib/git.py
+++ b/stgit/lib/git.py
@@ -579,10 +579,10 @@ class DiffTreeProcesses(object):
         p.stdin.write(query + end)
         p.stdin.flush()
         s = ''
-        while not (s.endswith('\n' + end) or s.endswith('\0' + end)):
+        while not s.endswith(end):
             s += os.read(p.stdout.fileno(), 4096)
-        assert s.startswith(query)
-        assert s.endswith(end)
+        if not s.startswith(query):
+            raise RepositoryException(os.read(p.stderr.fileno(), 4096))
         return s[len(query):-len(end)]
 
 class Repository(RunWithEnv):