Respect core.abbrev when abbreviating SHAs

[maninak]

We should respect core.abbrev when abbreviating git SHAs and keep defaulting to 7 chars if it isn't defined. This would improve support for non-small projects.

Theoretically, all we have to do is read the related git config and use it inside shortenHash().

Note, we should also respect special non-numeric values where core.abbrev can
be legally set to "auto" or "no", where "auto" gets an appropriate
length based on a loose count of objects in the current repo, and
"no" which doesn't abbreviate the SHA1 at all.

[lorenzleutgeb]

Proof of concept (you can execute this locally and verify that it prints the same as git rev-parse --short HEAD | tr -d '\n' | wc -c): https://gist.github.com/lorenzleutgeb/62a19762e5e7546ecfcdae2fb9b7d82d

Note in particular that the default for core.abbrev is not 7, but "auto"!