gitweb/Makefile: clear up and de-duplicate the gitweb.{css,js} vars

Change the variable definitions for the $(GITWEB_CSS) and $(GITWEB_JS)
so that we have a clear separation between what we use as "in" files,
v.s. our "min" files. We can now make the appending to $(GITWEB_FILES)
unconditional, since $(GITWEB_{JS,CSS}) is either the "min" or
non-"min" version. This reduces the duplication within the file.

While we're at it let's initialize "GITWEB_JSLIB_FILES" as we normally
do with such variables.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>