preferring https: to http: in commit messages for gnu.org

[Paul Eggert <eggert@cs.ucla.edu>]

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I installed the attached into the default Git commit hooks to help remind us 
(and particularly to remind me, since I write a fair number of these things) 
that https: should be preferred to http: in commit-message URLs that point to 
gnu.org or fsf.org web sites.

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From 3c78960a4755d7bd583a0411d7089a0c13c0c64b Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@day>
Date: Sat, 14 Oct 2017 19:16:12 -0700
Subject: [PATCH] Encourage https: in commit messages

* CONTRIBUTE: Prefer https: when citing.
* build-aux/git-hooks/commit-msg:
Diagnose http: and ftp: URLs to FSF or GNU hosts.
---
 CONTRIBUTE                     |  3 +++
 build-aux/git-hooks/commit-msg | 10 ++++++++++
 2 files changed, 13 insertions(+)

diff --git a/CONTRIBUTE b/CONTRIBUTE
index 8995bb3..c7f3330 100644
--- a/CONTRIBUTE
+++ b/CONTRIBUTE
@@ -175,6 +175,9 @@ them right the first time, so here are guidelines for formatting them:
   bug number NNNNN in the debbugs database.  This string is often
   parenthesized, as in "(Bug#19003)".
 
+- When citing URLs, prefer https: to http: when either will do.  In
+  particular, gnu.org and fsf.org URLs should start with "https:".
+
 - Commit messages should contain only printable UTF-8 characters.
 
 - Commit messages should not contain the "Signed-off-by:" lines that
diff --git a/build-aux/git-hooks/commit-msg b/build-aux/git-hooks/commit-msg
index 3945086..e21aaf0 100755
--- a/build-aux/git-hooks/commit-msg
+++ b/build-aux/git-hooks/commit-msg
@@ -66,6 +66,8 @@ at_sign=
         non_print = "[^[:print:]]"
       }
     }
+    c_lower = "abcdefghijklmnopqrstuvwxyz"
+    unsafe_gnu_url = "(http|ftp)://([" c_lower ".]*\\.)?(gnu|fsf)\\.org"
   }
 
   /^#/ {
@@ -125,6 +127,14 @@ at_sign=
     status = 1
   }
 
+  {
+    if (match($0, unsafe_gnu_url)) {
+      url = substr($0, RSTART, RLENGTH)
+      printf "Use https: URL instead of '\''%s'\'' in commit message\n", url
+      status = 1
+    }
+  }
+
   $0 ~ non_print {
     print "Unprintable character in commit message"
     status = 1
-- 
2.7.4