cmd/go/internal/vcs: prevent Git signatures from breaking commit time parsing

When a user has showSignature=true set in their Git config and the
commit in question has a signature, the git-show command will output
information about that signature. When this happens, the logic that
tries to parsing a timestamp from the git-show output chokes on this
signature information and the build stamping fails. This change prevents
commit signature information from being displayed even if
showSignature=true, preventing this issue.

Change-Id: I98d0a6fdd1e90dd1b91e0394713b6eb286a69d1a
GitHub-Last-Rev: 610706e23e33a037b9abede2ba0a926c0f336814
GitHub-Pull-Request: golang/go#49790
Reviewed-on: https://go-review.googlesource.com/c/go/+/367034


Trust: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>