maint: prefer https: to git:

The idea is to defend against some adversary-in-the-middle attacks.

.gitmodules

@@ -1,3 +1,3 @@
 [submodule "gnulib"]
         path = gnulib
-        url = git://git.sv.gnu.org/gnulib.git
+        url = https://git.savannah.gnu.org/git/gnulib.git

HACKING

@@ -20,7 +20,7 @@ Use the latest upstream sources
 Base any changes you make on the latest upstream sources.
 You can get a copy of the latest with this command:
 
-    git clone git://git.sv.gnu.org/coreutils
+    git clone https://git.savannah.gnu.org/git/coreutils.git
     cd coreutils
 
 That downloads the entire repository, including revision control history
@@ -95,7 +95,7 @@ Make your changes on a private "topic" branch
 =============================================
 So you checked out coreutils like this:
 
-  git clone git://git.sv.gnu.org/coreutils
+  git clone https://git.savannah.gnu.org/git/coreutils.git
 
 Now, cd into the coreutils/ directory and run:
 

README-hacking

@@ -27,7 +27,7 @@ few prerequisites, later, a plain 'git pull && make' typically suffices.
 
 You can get a copy of the source repository like this:
 
-        $ git clone git://git.sv.gnu.org/<packagename>
+        $ git clone https://git.savannah.gnu.org/git/<packagename>
         $ cd <packagename>
 
 where '<packagename>' stands for 'coreutils' or whatever other package