pathspec: allow escaped query values

In our own .gitattributes file we have attributes such as:

    *.[ch] whitespace=indent,trail,space

When querying for attributes we want to be able to ask for the exact
value, i.e.

    git ls-files :(attr:whitespace=indent,trail,space)

should work, but the commas are used in the attr magic to introduce
the next attr, such that this query currently fails with

fatal: Invalid pathspec magic 'trail' in ':(attr:whitespace=indent,trail,space)'

This change allows escaping characters by a backslash, such that the query

    git ls-files :(attr:whitespace=indent\,trail\,space)

will match all path that have the value "indent,trail,space" for the
whitespace attribute. To accomplish this, we need to modify two places.
First `parse_long_magic` needs to not stop early upon seeing a comma or
closing paren that is escaped. As a second step we need to remove any
escaping from the attr value.

Based on a patch by Stefan Beller <sbeller@google.com>

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6135-pathspec-with-attrs.sh

@@ -178,4 +178,23 @@ test_expect_success 'abort on asking for wrong magic' '
 	test_must_fail git ls-files . ":(attr:!label=foo)"
 '
 
+test_expect_success 'check attribute list' '
+	cat <<-EOF >>.gitattributes &&
+	* whitespace=indent,trail,space
+	EOF
+	git ls-files ":(attr:whitespace=indent\,trail\,space)" >actual &&
+	git ls-files >expect &&
+	test_cmp expect actual
+'
+
+test_expect_success 'backslash cannot be the last character' '
+	test_must_fail git ls-files ":(attr:label=foo\\ labelA=bar)" 2>actual &&
+	test_i18ngrep "not allowed as last character in attr value" actual
+'
+
+test_expect_success 'backslash cannot be used as a value' '
+	test_must_fail git ls-files ":(attr:label=f\\\oo)" 2>actual &&
+	test_i18ngrep "for value matching" actual
+'
+
 test_done