Provide mechanism to configure length of git commit abbrev

[serhiikartashov]

The plugin provides a way to retrieve the abbrev property which has a length of 7 symbols only (hardcoded).

it would be useful to configure the length of the abbrev property. In my situation, I want to set 8 symbols.
During quick research, I found that the dependent library was updated a few years ago and can provide a way to set length.
More details here: ajoberstar/grgit#254