string_list: add a function string_list_longest_prefix()

Add a function that finds the longest string from a string_list that
is a prefix of a given string.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-string-list.txt

@@ -75,6 +75,14 @@ Functions
 	to be deleted.  Preserve the order of the items that are
 	retained.
 
+`string_list_longest_prefix`::
+
+	Return the longest string within a string_list that is a
+	prefix (in the sense of prefixcmp()) of the specified string,
+	or NULL if no such prefix exists.  This function does not
+	require the string_list to be sorted (it does a linear
+	search).
+
 `print_string_list`::
 
 	Dump a string_list to stdout, useful mainly for debugging purposes. It