Make some strings const

Signed-off-by: Timo Hirvonen <tihirvon@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/sha1_file.c b/sha1_file.c
index c80528b..8179630 100644
--- a/sha1_file.c
+++ b/sha1_file.c
@@ -343,7 +343,7 @@
 
 void prepare_alt_odb(void)
 {
-	char *alt;
+	const char *alt;
 
 	alt = getenv(ALTERNATE_DB_ENVIRONMENT);
 	if (!alt) alt = "";