Better handling of multiple branches

[pawamoy]

Is your feature request related to a problem? Please describe.
In repositories with multiple branches, git log is not enough to get all the tags. Sometimes tags only appear in release branches.

Describe the solution you'd like
Add an option to work on every branches. This is possible with the --all flag of git log.

Describe alternatives you've considered
Checking out specific branches to run git-changelog. This is very limited.

Additional context
When working on all commits (git log --all), we must ensure the following things when building the versions:

• versions must be sorted semantically, not chronologically. For example, given versions 1, 2, 1.1 and 3 were published in that order, the versions must be chained as 3 -> 2 -> 1.1 -> 1, not 3 -> 1.1 -> 2 -> 1.
• versions must be rendered chronologically: if we just published a patch to an old version, it should still appear at the top of the changelog, not lower, and be compared against its semantic previous version.

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