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/ex_cmds2.c

@@ -471,7 +471,7 @@ dbg_parsearg (
   else if (
     gap != &prof_ga &&
     VIM_ISDIGIT(*p)) {
-    bp->dbg_lnum = getdigits(&p);
+    bp->dbg_lnum = get_long_digits(&p);
     p = skipwhite(p);
   } else
     bp->dbg_lnum = 0;