net: copy the option-injection detection from git

This function returns true if the string starts with a `-` as that could be used
to inject options into commands we execute.

src/util/net.c

@@ -1152,3 +1152,8 @@ void git_net_url_dispose(git_net_url *url)
 	git__free(url->username); url->username = NULL;
 	git__free(url->password); url->password = NULL;
 }
+
+int git_net_looks_like_command_line_option(const char *str)
+{
+	return str && str[0] == '-';
+}

src/util/net.h

@@ -107,4 +107,13 @@ extern bool git_net_url_matches_pattern_list(
 /** Disposes the contents of the structure. */
 extern void git_net_url_dispose(git_net_url *url);
 
+
+/**
+ * Returns true if the string starts with a dash
+ *
+ * These could be used to try to trick an executed subcommand like ssh to do
+ * something other than what we intend.
+ */
+int git_net_looks_like_command_line_option(const char *str);
+
 #endif