Add git commit hooks that do some simple checks on commits.

* autogen.sh: Install Git hooks, if using Git.
* build-aux/git-hooks/commit-msg, build-aux/git-hooks/pre-commit:
New files, which are Git hooks that check for portable file names, and
do some simple checks for commit message format.

build-aux/git-hooks/commit-msg

@@ -0,0 +1,90 @@
+#!/bin/sh
+# Check the format of GNU Emacs change log entries.
+
+# Copyright 2014 Free Software Foundation, Inc.
+
+# This file is part of GNU Emacs.
+
+# GNU Emacs is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# GNU Emacs is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
+
+# Written by Paul Eggert.
+
+# Use a UTF-8 locale if available, so that the UTF-8 check works.
+# Use U+00A2 CENT SIGN to test whether the locale works.
+cent_sign_utf8_octal='\302\242'
+at_sign=`
+  printf "${cent_sign_utf8_octal}@" |
+  awk '{print substr($0, 2)}' 2>/dev/null
+`
+if test "$at_sign" != @; then
+  at_sign=`
+    printf "${cent_sign_utf8_octal}@" |
+    LC_ALL=en_US.utf8 awk '{print substr($0, 2)}' 2>/dev/null
+  `
+  if test "$at_sign" = @; then
+    LC_ALL=en_US.utf8; export LC_ALL
+  fi
+fi
+
+# Check the log entry.
+exec awk '
+  /^#/ { next }
+
+  !/^.*$/ {
+    print "Invalid character (not UTF-8)"
+    status = 1
+  }
+
+  nlines == 0 && !/[^[:space:]]/ { next }
+
+  { nlines++ }
+
+  nlines == 1 && /^[[:space:]]/ {
+    print "White space at start of first line"
+    status = 1
+  }
+
+  nlines == 2 && /[^[:space:]]/ {
+    print "Nonempty second line"
+    status = 1
+  }
+
+  /[[:cntrl:]]/ {
+    print "Text contains control character; please use spaces instead of tabs"
+    status = 1
+  }
+
+  72 < length && /[[:space:]]/ {
+    print "Line longer than 72 characters"
+    status = 1
+  }
+
+  140 < length {
+    print "Word longer than 140 characters"
+    status = 1
+  }
+
+  /^Signed-off-by: / {
+    print "'Signed-off-by:' present"
+    status = 1
+  }
+
+  END {
+    if (nlines == 0) {
+      print "Empty change log entry"
+      status = 1
+    }
+    exit status
+  }
+' <"$1"