Git branches can become very long (e.g. gitlab auto-generated branch
names), thus it would be nice to be able to truncate them to keep your
prompt lenght in line.
This patch adds two new options to the git_branch module:
* truncation_length: The amount of graphemes to of a gitbranch to
truncate to
* truncation_symbol: The symbol that should be used to indicate that a
branch name was trunctated
To be able to correctly work with UTF-8 graphemes, unicode-segmentation
was added as a dependency.
