Don't try to parse signature check as commit timestamp

When the log.showSignature git setting is enabled, the output of
"git log" contains signature verification information in addition to the
timestamp GitInputScheme::fetch wants:

  $ git log -1 --format=%ct
  gpg: Signature made Sat 07 Sep 2019 02:02:03 PM PDT
  gpg:                using RSA key 0123456789ABCDEF0123456789ABCDEF01234567
  gpg:                issuer "user@example.com"
  gpg: Good signature from "User <user@example.com>" [ultimate] 1567890123
  1567890123

For folks that had log.showSignature set, this caused all nix operations
on flakes to fail:

  $ nix build
  error: stoull

0 files changed, 0 insertions, 0 deletions