Merge branch 'br/commit-tree-fully-spelled-gpg-sign-option'

The documentation of "git commit-tree" said that the command
understands "--gpg-sign" in addition to "-S", but the command line
parser did not know about the longhand, which has been corrected.

* br/commit-tree-fully-spelled-gpg-sign-option:
  commit-tree: add missing --gpg-sign flag
  t7510: invoke git as part of &&-chain