branchmap: remove use of buffer() to support Python 2.6
The use of buffer() was added in 7359157b9e46 to support Python 2.6,
which we no longer support.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a