clone: abort if no dir name could be guessed

Due to various components of the URI being stripped off it may
happen that we fail to guess a directory name. We currently error
out with a message that it is impossible to create the working
tree '' in such cases. Instead, error out early with a sensible
error message hinting that a directory name should be specified
manually on the command line.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/clone.c

@@ -215,6 +215,10 @@ static char *guess_dir_name(const char *repo, int is_bundle, int is_bare)
 	len = end - start;
 	strip_suffix_mem(start, &len, is_bundle ? ".bundle" : ".git");
 
+	if (!len || (len == 1 && *start == '/'))
+	    die("No directory name could be guessed.\n"
+		"Please specify a directory on the command line");
+
 	if (is_bare)
 		dir = xstrfmt("%.*s.git", (int)len, start);
 	else