git-qt-merge: Use #!/usr/bin/env instead of #!/usr/bin/python3

It prevents the script to fail if python3 is not installed.
Also workaround the use of raw_input() with python3.

Change-Id: If9d95ef3a5ffbd6ad29521f2b81c4622c6e51d4f
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>