Expose git_refspec_parse()

This function has been available for some time, but never in a
header. Expose it so we can use it from outside the library.

include/git2/refspec.h

@@ -19,6 +19,14 @@
  */
 GIT_BEGIN_DECL
 
+/**
+ * Parse a refspec string and create a refspec object
+ *
+ * @param refspec pointer to the refspec structure to be used
+ * @param str the refspec as a string
+ */
+GIT_EXTERN(int) git_refspec_parse(git_refspec *refspec, const char *str);
+
 /**
  * Get the source specifier
  *