git: add .gitattributes file (#13947)

Add a line to .gitattributes so that `git grep -p` shows function names
properly for `*.py` files.  Without this, the class name is shown instead
of the function for python files.

This also causes diff output to use proper functions as hunk headers in
`diff` output.

Here's an example with `git grep -p`.

Before:

    $ git grep -p spack_cc var/spack/repos/builtin/packages/athena
    var/spack/repos/builtin/packages/athena/package.py=class Athena(AutotoolsPackage):
    var/spack/repos/builtin/packages/athena/package.py:            env.set('CC', spack_cc)
    var/spack/repos/builtin/packages/athena/package.py:            env.set('LDR', spack_cc)

After:

    $ git grep -p spack_cc var/spack/repos/builtin/packages/athena
    var/spack/repos/builtin/packages/athena/package.py=    def setup_build_environment(self, env):
    var/spack/repos/builtin/packages/athena/package.py:            env.set('CC', spack_cc)
    var/spack/repos/builtin/packages/athena/package.py:            env.set('LDR', spack_cc)

Here's an example with `diff`.

Before:

    $ git show c5da94eb58
    [...]
    @@ -28,6 +29,7 @@ print(u'\\xc3')

             # make it executable
             fs.set_executable(script_name)
    +        filter_shebangs_in_directory('.', [script_name])

             # read the unicode back in and see whether things work
             script = ex.Executable('./%s' % script_name)

After:

    $ git show c5da94eb58
    [...]
    @@ -28,6 +29,7 @@ def test_read_unicode(tmpdir):

             # make it executable
             fs.set_executable(script_name)
    +        filter_shebangs_in_directory('.', [script_name])

             # read the unicode back in and see whether things work
             script = ex.Executable('./%s' % script_name)

.gitattributes

@@ -0,0 +1 @@
+*.py diff=python