git-var: constness and globalness cleanup.

var.c::git_var read function did not have to return writable
strings; make it and the functions it points at return const char *
instead.

ident.c::get_ident() did not need to be global, so make it
static.

Signed-off-by: Junio C Hamano <junkio@cox.net>

var.c

@@ -12,7 +12,7 @@ static const char var_usage[] = "git-var [-l | <variable>]";
 
 struct git_var {
 	const char *name;
-	char *(*read)(void);
+	const char *(*read)(void);
 };
 static struct git_var git_vars[] = {
 	{ "GIT_COMMITTER_IDENT", git_committer_info },