BUG: concat/append misc fixes
closes #13626 
closes #7795

whoosh! you really cleaned this up!

had a quick look, but will look deeper later.
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13660?src=pr) is 85.26% (diff: 97.05%)

> Merging [#13660](https://codecov.io/gh/pydata/pandas/pull/13660?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will decrease coverage by **<.01%**

``` diff
@@             master     #13660   diff @@
==========================================
  Files           139        139          
  Lines         50562      50506    -56   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
- Hits          43116      43066    -50   
+ Misses         7446       7440     -6   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [59524af...85f5d94](https://codecov.io/gh/pydata/pandas/compare/59524af1e90d3dda2d885e711f7258704a020b6d...85f5d9456ebd6c44d84b8def2a515cba62ecf51f?src=pr)
 status?
 you seem to fix a pivot name propogation issue. is there an issue?
 you must have just rebased this!

will look tom.
 Yeah let me work on this first, I'll include the cleanup for `CategoricalIndex` in #13767. 

Let you know once ready.
 @jreback updated, and ready for review.
 @sinhrks looks really nice. just some minor comments and guaranetees that should specify a bit more / use asserts. ping on green.
