gnu: git: Install the Bash completion script.

* gnu/packages/version-control.scm (git)[arguments]: Add
  'install-shell-completion' phase.

gnu/packages/version-control.scm

@@ -146,6 +146,16 @@ as well as the classic centralized workflow.")
             (("/bin/sh") (which "sh"))
             (("/usr/bin/perl") (which "perl"))
             (("/usr/bin/python") (which "python"))))
+        (alist-cons-after
+         'install 'install-shell-completion
+         (lambda* (#:key outputs #:allow-other-keys)
+           (let* ((out         (assoc-ref outputs "out"))
+                  (completions (string-append out "/etc/bash_completion.d")))
+             ;; TODO: Install the tcsh and zsh completions in the right place.
+             (mkdir-p completions)
+             (copy-file "contrib/completion/git-completion.bash"
+                        (string-append completions "/git.sh"))
+             #t))
          (alist-cons-after
           'install 'split
           (lambda* (#:key inputs outputs #:allow-other-keys)
@@ -203,7 +213,7 @@ as well as the classic centralized workflow.")
               (wrap-program (string-append out "/bin/git")
                 `("PATH" ":" prefix
                   ("$HOME/.guix-profile/libexec/git-core")))))
-         %standard-phases))))
+          %standard-phases)))))
    (synopsis "Distributed version control system")
    (description
     "Git is a free distributed version control system designed to handle