fix: look for .git dir in parents of input dir

[nexdrew]

PR #2 changed the git command used by next-build-id from using the -C flag to using the --git-dir and --work-tree flags, which are more strict in the sense that if the directory path given does not exist, git will fail, whereas the -C flag will work for any subdirectory within a git project.

Unfortunately the test suite for next-build-id was not smart enough to accurately mimic this git behavior, so the problem was unknown until I started using an upgraded version of this lib.

This PR fixes that regression by:

1. Changing the logic that issues the git command to first find the parent directory that contains a valid .git dir (if it exists) before calling the git command
2. Improving the mocked git command for tests such that it will fail if the --git-dir path does not exist