Add a gitignore coherency test

This tests helps to maintain .gitignore up to date, by :
 - testing if all ignored files have to be ignored, and
 - testing if all untracked files are ignored.

[ Note : second version ]
Make test work on older git versions
I.e. the git version used on Debian stable.

.gitignore

@@ -1,6 +1,5 @@
-# Note! Please use 'git ls-files -i --exclude-standard'
-# command after changing this file, to see if there are
-# any tracked files which get ignored after the change.
+# Note! Please run 'prove t/gitignore.t' after changing
+# this file, to check its coherency
 
 # all dotfiles
 .*

t/gitignore.t

@@ -0,0 +1,45 @@
+#!/usr/bin/perl -w
+#
+#  Test that .gitignore is coherent
+#
+# Stéphane (kwisatz) Jourdois
+# --
+#
+
+use strict;
+use Test::More tests => 3;
+
+BEGIN { use_ok( 'Git' ); }
+
+
+# First, check that no tracked files are ignored
+my $cmd = Git::command_output_pipe('ls-files', '--ignored', '--exclude-standard');
+my $output;
+while (<$cmd>) { $output .= "--> $_" }
+close $cmd;
+
+ok(!defined $output, 'No tracked file is ignored')
+    or diag(<<EOF
+
+Check that the following tracked files _have_to_ be ignored, and then either :
+    - 'git rm' them
+    - modify .gitignore to not ignore them
+EOF
+    . $output . "\n");
+
+
+# Now, check that no untracked files are present
+$cmd = Git::command_output_pipe('ls-files', '--others', '--exclude-standard');
+undef $output;
+while (<$cmd>) { $output .= "--> $_" }
+close $cmd;
+
+ok(!defined $output, 'No untracked file is present')
+    or diag(<<EOF
+
+Check whether the following untracked files have to be ignored or
+tracked, and either :
+    - 'git add' them
+    - modify .gitignore to ignore them
+EOF
+    . $output . "\n");