Move split_cmdline() to alias.c

split_cmdline() is currently used for aliases only, but later it can be
useful for other builtins as well. Move it to alias.c for now,
indicating that originally it's for aliases, but we'll have it in libgit
this way.

Signed-off-by: Miklos Vajna <vmiklos@frugalware.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -831,5 +831,6 @@ int report_path_error(const char *ps_matched, const char **pathspec, int prefix_
 void overlay_tree_on_cache(const char *tree_name, const char *prefix);
 
 char *alias_lookup(const char *alias);
+int split_cmdline(char *cmdline, const char ***argv);
 
 #endif /* CACHE_H */