Add function to resolve short commit ID to full SHA1

[lafriks]

Needed for go-gitea/gitea#4127

[lunny]

LGTM

[daviian]

@lafriks Can you add a test verifying that it really outputs "exit status 128" if the rev does not exist? My local git doesn't return that.

[lafriks]

@daviian added tests for both cases (found and not found).
When running from console it will not contain exact text exit status 128, it will be just that exit status is 128. That I verified and works as expected.