Move declaration for alias.c to alias.h

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1835,11 +1835,6 @@ extern int ws_blank_line(const char *line, int len, unsigned ws_rule);
 void overlay_tree_on_index(struct index_state *istate,
 			   const char *tree_name, const char *prefix);
 
-char *alias_lookup(const char *alias);
-int split_cmdline(char *cmdline, const char ***argv);
-/* Takes a negative value returned by split_cmdline */
-const char *split_cmdline_strerror(int cmdline_errno);
-
 /* setup.c */
 struct startup_info {
 	int have_repository;