mq: use fallback patch name if no alpha-numeric in summary line (issue5025)

Before this patch, "hg qimport -r REV" fails, if the summary line of
description of REV doesn't contain any alpha-numeric bytes.

In this case, all bytes in the summary line 'title' are dropped from
'namebase' by the code path below.

     namebase = re.sub('[\s\W_]+', '_', title.lower()).strip('_')

'makepatchname()' immediately returns this empty string as valid patch
name, because patch name conflicting against empty string never
exists.

Then, "hg qimport -r REV" is aborted at creation of patch file with
empty filename.

This situation isn't so rare. For example, ordinary texts in Japanese
often consist of non alpha-numeric bytes in UTF-8.

This patch makes 'makepatchname()' use fallback patch name if the
summary line of imported revision doesn't contain any alpha-numeric
bytes.

hgext/mq.py

@@ -395,10 +395,12 @@ def newcommit(repo, phase, *args, **kwargs):
 class AbortNoCleanup(error.Abort):
     pass
 
-def makepatchname(existing, title):
+def makepatchname(existing, title, fallbackname):
     """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('_')
+    if not namebase:
+        namebase = fallbackname
     name = namebase
     i = 0
     while name in existing:
@@ -2101,7 +2103,8 @@ class queue(object):
 
                     if not patchname:
                         patchname = makepatchname(self.fullseries,
-                            repo[r].description().split('\n', 1)[0])
+                            repo[r].description().split('\n', 1)[0],
+                            '%d.diff' % r)
                     checkseries(patchname)
                     self.checkpatchname(patchname, force)
                     self.fullseries.insert(0, patchname)

tests/test-mq-qimport.t

@@ -290,3 +290,26 @@ check qimport phase:
   $ cd ..
 
   $ killdaemons.py
+
+check patch name generation for non-alpha-numeric summary line
+
+  $ cd repo
+
+  $ hg qpop -a -q
+  patch queue now empty
+  $ hg qseries -v
+  0 U imported_patch_b_diff
+  1 U 0
+  2 U this-name-is-better
+  3 U url.diff
+
+  $ echo bb >> b
+  $ hg commit -m '==++--=='
+
+  $ hg qimport -r tip
+  $ hg qseries -v
+  0 A 1.diff
+  1 U imported_patch_b_diff
+  2 U 0
+  3 U this-name-is-better
+  4 U url.diff