Remove magic hack

It should be assumed that on modern development environments, python
is accessible to /usr/bin/env

Change the shebang as necessary and remove the magic hack.

This also means losing the -E option on the call to python, so that
PYTHONPATH and PYTHONHOME will be respected and local configuration
problems in those vars would be noticed

Change-Id: I6f0708ca7693f05a4c3621c338f03619563ba630
diff --git a/repo b/repo
index 80eff8f..96302c0 100755
--- a/repo
+++ b/repo
@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env python
 
 ## repo default configuration
 ##
@@ -19,14 +19,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-magic='--calling-python-from-/bin/sh--'
-"""exec" python -E "$0" "$@" """#$magic"
-if __name__ == '__main__':
-  import sys
-  if sys.argv[-1] == '#%s' % magic:
-    del sys.argv[-1]
-del magic
-
 # increment this whenever we make important changes to this script
 VERSION = (1, 19)