Cleanup: Refactor getdigits().

Problem  : getdigits() currently returns a long, but at most places,
           return value is casted (unsafely) into an int. Making casts
           safe would introduce a lot of fuss in the form of assertions
           checking for limits.
Note     : We cannot just change return type to int, because, at some
           places, legitimate long values are used. For example, in
           diff.c, for line numbers.
Solution : Introduce new functions:
           - get_digits()      : Gets an intmax_t from a string.
           - get_int_digits()  : Wrapper for ints.
           - get_long_digits() : Wrapper for longs.
           And replace getdigits() invocations by the appropiate
           wrapper invocations.

src/nvim/digraph.c

@@ -1611,9 +1611,7 @@ void putdigraph(char_u *str)
       EMSG(_(e_number_exp));
       return;
     }
-    int64_t digits = getdigits(&str);
-    assert(digits <= INT_MAX);
-    int n = (int)digits;
+    int n = get_int_digits(&str);
 
     // If the digraph already exists, replace the result.
     dp = (digr_T *)user_digraphs.ga_data;