Fix missing close in WalkGitLog (#17008)

Fix missing close in WalkGitLog (#17008)

When the external context is cancelled it is possible for the
GitLogReader to not itself be Closed.

This PR does three things:

1. Instead of adding a plain defer it wraps the `g.Close` in a func as
`g` may change.
2. It adds the missing explicit g.Close - although the defer fix makes
this unnecessary.
3. It passes down the external context as the base context for the
GitLogReader meaning that the cancellation of the external context will
pass down automatically.

Fix #17007

Signed-off-by: Andrew Thornton <art27@cantab.net>