branchmap: add seek() to end of file before calling tell() on append open()

branchmap: add seek() to end of file before calling tell() on append open()

This is similar to 48c232873a54, which was subsequently modified in 19f5dec2d61f
for 2.4.  Unexpected test changes on Windows occurred without this.

--- a/mercurial/branchmap.py	Fri Jan 09 22:14:01 2015 -0500
+++ b/mercurial/branchmap.py	Sat Jan 10 12:00:03 2015 -0500
@@ -407,6 +407,9 @@
             try:
                 if self._rbcnamescount != 0:
                     f = repo.vfs.open(_rbcnames, 'ab')
+                    # The position after open(x, 'a') is implementation defined-
+                    # see issue3543.  SEEK_END was added in 2.5
+                    f.seek(0, 2) #os.SEEK_END
                     if f.tell() == self._rbcsnameslen:
                         f.write('\0')
                     else:
@@ -431,6 +434,9 @@
                                    len(self._rbcrevs) // _rbcrecsize)
             try:
                 f = repo.vfs.open(_rbcrevs, 'ab')
+                # The position after open(x, 'a') is implementation defined-
+                # see issue3543.  SEEK_END was added in 2.5
+                f.seek(0, 2) #os.SEEK_END
                 if f.tell() != start:
                     f.seek(start)
                     f.truncate()