refactor dir_add_name

This is in preparation for keeping two entry lists in the
dir object.

This patch adds and uses the ALLOC_GROW() macro, which
implements the commonly used idiom of growing a dynamic
array using the alloc_nr function (not just in dir.c, but
everywhere).

We also move creation of a dir_entry to dir_entry_new.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -225,6 +225,21 @@ extern void verify_non_filename(const char *prefix, const char *name);
 
 #define alloc_nr(x) (((x)+16)*3/2)
 
+/*
+ * Realloc the buffer pointed at by variable 'x' so that it can hold
+ * at least 'nr' entries; the number of entries currently allocated
+ * is 'alloc', using the standard growing factor alloc_nr() macro.
+ *
+ * DO NOT USE any expression with side-effect for 'x' or 'alloc'.
+ */
+#define ALLOC_GROW(x, nr, alloc) \
+	do { \
+		if ((nr) >= alloc) { \
+			alloc = alloc_nr(alloc); \
+			x = xrealloc((x), alloc * sizeof(*(x))); \
+		} \
+	} while(0)
+
 /* Initialize and use the cache information */
 extern int read_index(struct index_state *);
 extern int read_index_from(struct index_state *, const char *path);