Add "snapshot-prefix" repo configuration

Allow using a user-specified value for the prefix in snapshot files
instead of the repository basename.  For example, files downloaded from
the linux-stable.git repository should be named linux-$VERSION and not
linux-stable-$VERSION, which can be achieved by setting:

	repo.snapshot-prefix=linux

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>
diff --git a/ui-shared.c b/ui-shared.c
index e719c1b..d857873 100644
--- a/ui-shared.c
+++ b/ui-shared.c
@@ -152,6 +152,14 @@
 	return rvbuf;
 }
 
+const char *cgit_snapshot_prefix(const struct cgit_repo *repo)
+{
+	if (repo->snapshot_prefix)
+		return repo->snapshot_prefix;
+
+	return cgit_repobasename(repo->url);
+}
+
 static void site_url(const char *page, const char *search, const char *sort, int ofs, int always_root)
 {
 	char *delim = "?";
@@ -1110,7 +1118,7 @@
 	struct strbuf filename = STRBUF_INIT;
 	size_t prefixlen;
 
-	cgit_compose_snapshot_prefix(&filename, cgit_repobasename(repo->url), hex);
+	cgit_compose_snapshot_prefix(&filename, cgit_snapshot_prefix(repo), hex);
 	prefixlen = filename.len;
 	for (f = cgit_snapshot_formats; f->suffix; f++) {
 		if (!(repo->snapshots & f->bit))