patch: correctly handle non-tabular Subject: line

The line content of continued Subject: lines was yet joined via
str.replace('\n\t', ' '), which does not handle continuation via
spaces.  So expan the regular expression instead to
handle all allowed forms of mail header line continuation.

mercurial/patch.py

@@ -188,7 +188,7 b' def extract(ui, fileobj):'
                 pend = subject.find(']')
                 if pend >= 0:
                     subject = subject[pend + 1:].lstrip()
-            subject = subject.replace('\n\t', ' ')
+            subject = re.sub(r'\n[ \t]+', ' ', subject)
             ui.debug('Subject: %s\n' % subject)
         if user:
             ui.debug('From: %s\n' % user)