ui-shared: Remove needless cgit_get_cmd() call, and refactor hc() accordingly

The call to cgit_get_cmd() and the following fallback handling (to "summary"
or "repoindex") in cgit_print_pageheader() is unnecessary, since the same
fallback handling was already done when ctx.qry.page was set when
cgit_get_cmd() was called from process_request() in cgit.c.

As such, hc() can also be rewritten to simply compare the given 'page' string
against ctx.qry.page.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>