export: clobber files with -o (bc) (issue3652)

This violated user expectation. Updated the code to clobber files, but
preserve the behavior of appending multiple patches requested in a
single export. Includes tests.

mercurial/cmdutil.py

@@ -170,7 +170,8 b' def makefilename(repo, pat, node, desc=N'
                          inst.args[0])
 
 def makefileobj(repo, pat, node=None, desc=None, total=None,
-                seqno=None, revwidth=None, mode='wb', pathname=None):
+                seqno=None, revwidth=None, mode='wb', modemap={},
+                pathname=None):
 
     writable = mode not in ('r', 'rb')
 
@@ -196,9 +197,11 b' def makefileobj(repo, pat, node=None, de'
         return pat
     if util.safehasattr(pat, 'read') and 'r' in mode:
         return pat
-    return open(makefilename(repo, pat, node, desc, total, seqno, revwidth,
-                              pathname),
-                mode)
+    fn = makefilename(repo, pat, node, desc, total, seqno, revwidth, pathname)
+    mode = modemap.get(fn, mode)
+    if mode == 'wb':
+        modemap[fn] = 'ab'
+    return open(fn, mode)
 
 def openrevlog(repo, cmd, file_, opts):
     """opens the changelog, manifest, a filelog or a given revlog"""
@@ -539,6 +542,7 b" def export(repo, revs, template='hg-%h.p"
 
     total = len(revs)
     revwidth = max([len(str(rev)) for rev in revs])
+    filemode = {}
 
     def single(rev, seqno, fp):
         ctx = repo[rev]
@@ -554,7 +558,8 b" def export(repo, revs, template='hg-%h.p"
             desc_lines = ctx.description().rstrip().split('\n')
             desc = desc_lines[0]    #Commit always has a first line.
             fp = makefileobj(repo, template, node, desc=desc, total=total,
-                             seqno=seqno, revwidth=revwidth, mode='ab')
+                             seqno=seqno, revwidth=revwidth, mode='wb',
+                             modemap=filemode)
             if fp != template:
                 shouldclose = True
         if fp and fp != sys.stdout and util.safehasattr(fp, 'name'):

tests/test-export.t

@@ -91,13 +91,28 b''
   foo-foo_10.patch
   foo-foo_11.patch
 
+Doing it again clobbers the files rather than appending:
+  $ hg export -v -o "foo-%m.patch" 2:3
+  exporting patches:
+  foo-foo_2.patch
+  foo-foo_3.patch
+  $ grep HG foo-foo_2.patch | wc -l
+  \s*1 (re)
+  $ grep HG foo-foo_3.patch | wc -l
+  \s*1 (re)
+
 Exporting 4 changesets to a file:
 
   $ hg export -o export_internal 1 2 3 4
   $ grep HG export_internal | wc -l
   \s*4 (re)
 
-Exporting 4 changesets to a file:
+Doing it again clobbers the file rather than appending:
+  $ hg export -o export_internal 1 2 3 4
+  $ grep HG export_internal | wc -l
+  \s*4 (re)
+
+Exporting 4 changesets to stdout:
 
   $ hg export 1 2 3 4 | grep HG | wc -l
   \s*4 (re)