errors: introduce EINVALID

We've been using EINVALIDSPEC for a while to mean this, but that name
is too specific. Introduce this to be more explicit.

include/git2/errors.h

@@ -46,6 +46,7 @@ typedef enum {
 	GIT_EAPPLIED        = -18,	/**< Patch/merge has already been applied */
 	GIT_EPEEL           = -19,      /**< The requested peel operation is not possible */
 	GIT_EEOF            = -20,      /**< Unexpected EOF */
+	GIT_EINVALID        = -21,      /**< Invalid operation or input */
 
 	GIT_PASSTHROUGH     = -30,	/**< Internal only */
 	GIT_ITEROVER        = -31,	/**< Signals end of iteration with iterator */