Simplify-by-decoration with decorate-refs-exclude

["Étienne SERVAIS" <etienne.servais@voucoux.fr>]

Hi there,

I've asked this question yesterday on stackoverflow: 
https://stackoverflow.com/q/57305719/2622010
And have been confirmed it does look like a bug. 
Unfortunately I'm unable to update to latest git revision but a search through the release notes didn't show anything related to this problem. 

I'm working in a git (v2.19.1) repo with lots of tags and branches. To get a glance of the git tree, I'd like to use the `--simplify-by-decoration` option while excluding some of the tags with `--decorate-refs-exclude=<pattern>` of `git log`, but, as per [the documentation](https://git-scm.com/docs/git-log/2.22.0#Documentation/git-log.txt---simplify-by-decoration):

> --simplify-by-decoration
> 
>     Commits that are referred by some branch or tag are selected.

Thus every tags are selected even those that are excluded from the decoration by the pattern.

Thus, when I enter

```
git log --oneline --graph  --decorate=full --decorate-refs-exclude='refs/tags/<pattern>'
```
The selected tags are properly excluded but once I add the `simplify-by-decoration` option

```
git log --oneline --graph  --decorate=full --decorate-refs-exclude='refs/tags/<pattern>' --simplify-by-decoration
```
The excluded tags pop back again.

Is there a workaround?

Étienne
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+33 6 45 98 22 65
Envoyé de mon appareil Android avec Courriel K-9 Mail. Veuillez excuser ma brièveté.