gitattributes: introduce and use "generated" attribute

I want to mark some files to be ignored for licensing purposes,
e.g. output from fuzzers and other samples. By using the gitattribute
machinery for this we don't need to design a custom protocol:

$ git check-attr generated test/test-sysusers/unhappy-*
test/test-sysusers/unhappy-1.expected-err: generated: set
test/test-sysusers/unhappy-1.input: generated: unspecified
test/test-sysusers/unhappy-2.expected-err: generated: set
test/test-sysusers/unhappy-2.input: generated: unspecified
test/test-sysusers/unhappy-3.expected-err: generated: set
test/test-sysusers/unhappy-3.input: generated: unspecified

.gitattributes

@@ -1,3 +1,11 @@
 *.[ch] whitespace=tab-in-indent,trailing-space
-*.gpg  binary
+*.gpg  binary generated
 *.bmp  binary
+
+# Mark files as "generated", i.e. no license applies to them.
+# This includes output from programs, directive lists generated by grepping
+# for all possibilities, samples from fuzzers, files from /proc, packet samples,
+# and anything else where no copyright can be asserted.
+#
+# Use 'git check-attr generated -- <path>' to query the attribute.
+[attr]generated

po/.gitattributes

@@ -0,0 +1 @@
+/LINGUAS  generated

test/dmidecode-dumps/.gitattributes

@@ -1 +1 @@
-/*.bin binary
+/*.bin binary generated

test/fuzz/.gitattributes

@@ -5,3 +5,4 @@
 /fuzz-fido-id-desc/ binary
 /fuzz-lldp-rx/*     binary
 /fuzz-ndisc-rs/*    binary
+/*/*                generated

test/journal-data/.gitattributes

@@ -1,2 +1,2 @@
 # Journal data in export format
-/*.txt  binary
+/*.txt  binary generated

test/test-network-generator-conversion/.gitattributes

@@ -0,0 +1 @@
+*  generated

test/test-network/.gitattributes

@@ -0,0 +1 @@
+/conf/*.key  generated

test/test-resolve/.gitattributes

@@ -1 +1 @@
-/*.pkts  binary
+/*.pkts  binary generated

test/test-sysusers/.gitattributes

@@ -0,0 +1,2 @@
+/*.initial*   generated
+/*.expected*  generated

test/test-umount/.gitattributes

@@ -0,0 +1 @@
+*  generated