viml/profile: revert proftime_T to unsigned type #10521

- reltimestr(): Produce negative value by comparing the unsigned
  proftime_T value to INT64_MAX.

https://github.com/neovim/neovim/issues/10452#issuecomment-511155132
1. The interfaces of nearly all platforms return uint64_t. INT64_MAX is
   only half of that.
2. Low-level interfaces like this typically define that there is no
   fixed starting point. The only guarantees are that it's (a)
   monotonically increasing at a rate that (b) matches real time.

ref 06af88cd72
fix #10452

test/functional/eval/reltime_spec.lua

@@ -38,9 +38,13 @@ describe('reltimestr(), reltimefloat()', function()
     local older_time = reltime()
     command('sleep 1m')
     local newer_time = reltime()
-    -- Should be something like -0.002123.
+
+    -- Start/end swapped: should be something like -0.002123.
     local rv = tonumber(reltimestr(reltime(newer_time, older_time)))
-    ok(rv < 0)
-    ok(rv > -10)
+    ok(rv < 0 and rv > -10)
+
+    -- Not swapped: should be something like 0.002123.
+    rv = tonumber(reltimestr(reltime(older_time, newer_time)))
+    ok(rv > 0 and rv < 10)
   end)
 end)