Add support for a "GIT_INDEX_FILE" environment variable.

We use that to specify alternative index files, which can be useful
if you want to (for example) generate a temporary index file to do
some specific operation that you don't want to mess with your main
one with.

It defaults to the regular ".git/index" if it hasn't been specified.

cache.h

@@ -11,6 +11,7 @@
 #include <string.h>
 #include <errno.h>
 #include <sys/mman.h>
+#include <sys/param.h>
 #include <netinet/in.h>
 
 #include <openssl/sha.h>
@@ -84,6 +85,13 @@ unsigned int active_nr, active_alloc;
 #define DB_ENVIRONMENT "SHA1_FILE_DIRECTORY"
 #define DEFAULT_DB_ENVIRONMENT ".git/objects"
 
+#define get_object_directory() (getenv(DB_ENVIRONMENT) ? : DEFAULT_DB_ENVIRONMENT)
+
+#define INDEX_ENVIRONMENT "GIT_INDEX_FILE"
+#define DEFAULT_INDEX_ENVIRONMENT ".git/index"
+
+#define get_index_file() (getenv(INDEX_ENVIRONMENT) ? : DEFAULT_INDEX_ENVIRONMENT)
+
 #define alloc_nr(x) (((x)+16)*3/2)
 
 /* Initialize and use the cache information */