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>

alias.h

@@ -0,0 +1,9 @@
+#ifndef __ALIAS_H__
+#define __ALIAS_H__
+
+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);
+
+#endif