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