add macro REALLOC_ARRAY

The macro ALLOC_GROW manages several aspects of dynamic memory
allocations for arrays: It performs overprovisioning in order to avoid
reallocations in future calls, updates the allocation size variable,
multiplies the item size and thus allows users to simply specify the
item count, performs the reallocation and updates the array pointer.

Sometimes this is too much.  Add the macro REALLOC_ARRAY, which only
takes care of the latter three points and allows users to specfiy the
number of items the array can store.  It can increase and also decrease
the size.  Using the macro avoid duplicating the variable name and
takes care of the item sizes automatically.
Signed-off-by: NRene Scharfe <l.s.r@web.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/technical/api-allocation-growing.txt

@@ -34,3 +34,6 @@ item[nr++] = value you like;
 ------------
 
 You are responsible for updating the `nr` variable.
+
+If you need to specify the number of elements to allocate explicitly
+then use the macro `REALLOC_ARRAY(item, alloc)` instead of `ALLOC_GROW`.

git-compat-util.h

@@ -626,6 +626,8 @@ extern int odb_mkstemp(char *template, size_t limit, const char *pattern);
 extern int odb_pack_keep(char *name, size_t namesz, const unsigned char *sha1);
 extern char *xgetcwd(void);
 
+#define REALLOC_ARRAY(x, alloc) (x) = xrealloc((x), (alloc) * sizeof(*(x)))
+
 static inline size_t xsize_t(off_t len)
 {
 	if (len > (size_t) len)