Adhere to PEP 394

From PEP 394 [1]:

* python2 will refer to some version of Python 2.x.

* end users should be aware that python refers to python3 on at least
  Arch Linux (that change is what prompted the creation of this PEP),
  so python should be used in the shebang line only for scripts that
  are source compatible with both Python 2 and 3.

So to make sure that we run correctly on a system where python refers
to python3 and avoid problems like issue #11 we change the shebangs.

[1] https://www.python.org/dev/peps/pep-0394/

hg-fast-export.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright (c) 2007, 2008 Rocco Rutte <pdmef@gmx.net> and others.
 # License: MIT <http://www.opensource.org/licenses/mit-license.php>

hg-fast-export.sh

@@ -26,7 +26,7 @@ SFX_MARKS="marks"
 SFX_HEADS="heads"
 SFX_STATE="state"
 GFI_OPTS=""
-PYTHON=${PYTHON:-python}
+PYTHON=${PYTHON:-python2}
 
 USAGE="[--quiet] [-r <repo>] [--force] [-m <max>] [-s] [--hgtags] [-A <file>] [-B <file>] [-T <file>] [-M <name>] [-o <name>] [--hg-hash] [-e <encoding>]"
 LONG_USAGE="Import hg repository <repo> up to either tip or <max>

hg2git.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python2
 
 # Copyright (c) 2007, 2008 Rocco Rutte <pdmef@gmx.net> and others.
 # License: MIT <http://www.opensource.org/licenses/mit-license.php>