Add support for snapshots

Make a link from the commit viewer to a snapshot of the corresponding tree.

Currently only zip-format is supported.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -6,7 +6,8 @@ gitsrc = ../git
 CACHE_ROOT = /var/cache/cgit
 EXTLIBS = $(gitsrc)/libgit.a $(gitsrc)/xdiff/lib.a -lz -lcrypto
 OBJECTS = shared.o cache.o parsing.o html.o ui-shared.o ui-repolist.o \
-	ui-summary.o ui-log.o ui-view.c ui-tree.c ui-commit.c ui-diff.o
+	ui-summary.o ui-log.o ui-view.c ui-tree.c ui-commit.c ui-diff.o \
+	ui-snapshot.o
 
 CFLAGS += -Wall