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/misc1.c

@@ -525,7 +525,7 @@ open_line (
           if (*p == COM_RIGHT || *p == COM_LEFT)
             c = *p++;
           else if (VIM_ISDIGIT(*p) || *p == '-')
-            off = getdigits(&p);
+            off = get_int_digits(&p);
           else
             ++p;
         }