patman: Detect missing upstream in CountCommitsToBranch

At present if we fail to find the upstream then the error output is piped
to wc, resulting in bogus results. Avoid the pipe and check the output
directly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/gitutil.py b/tools/patman/gitutil.py
index 27a0a9f..3a2366bc 100644
--- a/tools/patman/gitutil.py
+++ b/tools/patman/gitutil.py
@@ -66,9 +66,13 @@
         rev_range = '%s..%s' % (us, branch)
     else:
         rev_range = '@{upstream}..'
-    pipe = [LogCmd(rev_range, oneline=True), ['wc', '-l']]
-    stdout = command.RunPipe(pipe, capture=True, oneline=True).stdout
-    patch_count = int(stdout)
+    pipe = [LogCmd(rev_range, oneline=True)]
+    result = command.RunPipe(pipe, capture=True, capture_stderr=True,
+                             oneline=True, raise_on_error=False)
+    if result.return_code:
+        raise ValueError('Failed to determine upstream: %s' %
+                         result.stderr.strip())
+    patch_count = len(result.stdout.splitlines())
     return patch_count
 
 def NameRevision(commit_hash):