entry: extract update_ce_after_write() from write_entry()

The code that updates the in-memory index information after an entry is
written currently resides in write_entry(). Extract it to a public
function so that it can be called by the parallel checkout functions,
outside entry.c, in a later patch.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

entry.h

@@ -41,5 +41,7 @@ void unlink_entry(const struct cache_entry *ce);
 
 void *read_blob_entry(const struct cache_entry *ce, unsigned long *size);
 int fstat_checkout_output(int fd, const struct checkout *state, struct stat *st);
+void update_ce_after_write(const struct checkout *state, struct cache_entry *ce,
+			   struct stat *st);
 
 #endif /* ENTRY_H */