gitweb: Consolidate escaping/validation of query string

gitweb: Consolidate escaping/validation of query string

Consider:

	http://repo.or.cz/?p=glibc-cvs.git;a=tree;h=2609cb0411389325f4ee2854cc7159756eb0671e;hb=2609cb0411389325f4ee2854cc7159756eb0671e

(click on the funny =__ify file)

We ought to handle anything in filenames and I actually see no reason why
we don't, modulo very little missing escaping that this patch hopefully
also fixes.

I have also made esc_param() escape [?=&;]. Not escaping [&;] was downright
buggy and [?=] just feels better escaped. ;-) YMMV.

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