pager: explicitly cast away strchr() constness

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 <redacted>
Signed-off-by: Junio C Hamano <redacted>