py3: replace file() with open() in test-convert-git.t

file() is not present in Python 3.
It also makes sure we write bytes on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D2133

tests/test-convert-git.t

@@ -420,7 +420,7 b' test binary conversion (issue1359)'
   $ mkdir git-repo3
   $ cd git-repo3
   $ git init-db >/dev/null 2>/dev/null
-  $ $PYTHON -c 'file("b", "wb").write("".join([chr(i) for i in range(256)])*16)'
+  $ $PYTHON -c 'import struct; open("b", "wb").write(b"".join([struct.Struct(">B").pack(i) for i in range(256)])*16)'
   $ git add b
   $ commit -a -m addbinary
   $ cd ..
@@ -437,7 +437,7 b' convert binary file'
   $ cd git-repo3-hg
   $ hg up -C
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
-  $ $PYTHON -c 'print len(file("b", "rb").read())'
+  $ $PYTHON -c 'print len(open("b", "rb").read())'
   4096
   $ cd ..