mq: restrict generated patch name to 75 characters (issue5117)

Super long first line in description lead to very long file name that
windows is unhappy about. We restrict the name to 75 char to avoid the
issue.

75 seems fine and leave some extra room for '__#' suffix in case of conflict.

I does not seems worthwhile to add a dedicated config option to configure the
length. It can be done in the future if there is an actual user demand for it.

hgext/mq.py

@@ -1117,6 +1117,7 b' class queue(object):'
         """Return a suitable filename for title, adding a suffix to make
         it unique in the existing list"""
         namebase = re.sub('[\s\W_]+', '_', title.lower()).strip('_')
+        namebase = namebase[:75] # avoid too long name (issue5117)
         if namebase:
             try:
                 self.checkreservedname(namebase)

tests/test-mq-qimport.t

@@ -331,3 +331,12 b' check reserved patch names'
   2.diff
   taken__2
 
+check very long patch name
+
+  $ hg qpop -qa
+  patch queue now empty
+  $ echo >> b
+  $ hg commit -m 'abcdefghi pqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghi pqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghi pqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghi pqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghi pqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghi pqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
+  $ hg qimport -r .
+  $ hg qap
+  abcdefghi_pqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcdefghi_pqrstuvwxyzabcdefg