Install the shell completion scripts (#3784)

* Install the shell completion scripts

Install the `git-completion.{bash,tcsh,zsh}` and `git-prompt.sh` into
`$(spack location -i git)/share/` (aka `prefix.share/`).

* Use copy_tree to copy all the files

The dest dir already exists, so install_tree()'s not an option.

var/spack/repos/builtin/packages/git/package.py

@@ -24,6 +24,7 @@
 ##############################################################################
 import sys
 from spack import *
+from distutils.dir_util import copy_tree
 
 
 class Git(AutotoolsPackage):
@@ -166,6 +167,10 @@ def filter_rt(self):
             # Don't link with -lrt; the system has no (and needs no) librt
             filter_file(r' -lrt$', '', 'Makefile')
 
+    @run_after('install')
+    def install_completions(self):
+        copy_tree('contrib/completion', self.prefix.share)
+
     @run_after('install')
     def install_manpages(self):
         prefix = self.prefix