Option --release-branch seems to have no effect

First of all, thanks for sharing this tool! :- )

I'm now working with a repo that has tags in multiple branches:
```
v1.0.0 v1.1.0 v1.2.0
| | |
- - ---o----------o----------o master branch
\
\
o----------o hotfix/1.0.x branch
| |
v1.0.1 v1.0.2

-> time ->
```
However, I've noticed that passing different values to `--release-branch` seems to make no difference whatsoever. Basically, with either:
```
github_changelog_generator <...> --release-branch master
```
or
```
github_changelog_generator <...> --release-branch hotfix/1.0.x
```
the resulting changelog is exactly the same (all tags from all branches are included):
```
# Changelog

## [v1.1.2](https://github.com/.../.../tree/v1.1.2) (2020-05-29)

[Full Changelog](https://github.com/.../.../compare/v1.2.0...v1.1.2)
**Merged pull requests:**
...

## [v1.2.0](https://github.com/.../.../tree/v1.2.0) (2020-05-29)

[Full Changelog](https://github.com/.../.../compare/v1.1.1...v1.2.0)
**Merged pull requests:**
...

## [v1.1.1](https://github.com/.../.../tree/v1.1.1) (2020-05-29)

[Full Changelog](https://github.com/.../.../compare/v1.1.0...v1.1.1)
**Merged pull requests:**
...

## [v1.1.0](https://github.com/.../.../tree/v1.1.0) (2020-05-29)

[Full Changelog](https://github.com/.../.../compare/v1.0.0...v1.1.0)
**Merged pull requests:**
...

## [v1.0.0](https://github.com/.../.../tree/v1.0.0) (2020-05-26)

[Full Changelog](https://github.com/.../.../compare/aa0b48fdf8eb378ccc4bf7c228d7049396279619...v1.0.0)
**Merged pull requests:**
...
```