pager: explicitly cast away strchr() constness

When we do: char *cp = strchr(argv[i], '='); it implicitly removes the constness from argv[i]. We need "cp" to remain writable (since we overwrite it with a NUL). In theory we should be able to drop the const from argv[i], because it is a sub-pointer into our duplicated pager_env variable. But we get it from split_cmdline(), which uses the traditional "const char **" type for argv. This is overly limiting, but changing it would be awkward for all the other callers of split_cmdline(). Let's do an explicit cast with a note about why it is OK. This is enough to silence compiler warnings about the implicit const problems. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>