[BUG] git bisect colour output contrary to configuration

My ~/.gitconfig sets color.ui=never, which should prevent attempts at colouring output from all git commands. I do not have any git configuration enabling colour in any situation (such as for specific commands). But when a git bisect completes, the output identifying the first bad commit includes escape sequences to colour the "commit 3e6..." line yellow. Excerpt of strace output (with many irrelevant lines omitted):

23851 write(1, "3e6fc602e433dbd76941ac0ef7a438a77fbe9a05 is the first bad commit\n", 65) = 65 23851 open("/home/zefram/.gitconfig", O_RDONLY) = 3 23851 read(3, "[user]\n\tname = Zefram\n\temail = zefram@fysh.org\n\tsigningkey = 0x8E1E1EC1\n\n[color]\n\tui = never\n", 4096) = 93 23851 write(1, "\33[33mcommit 3e6fc602e433dbd76941ac0ef7a438a77fbe9a05\33[m\n", 56) = 56

Given the configuration, that line should be free of escape sequences.

I'm mainly using git 2.1.4 via Debian, but I've also reproduced this problem with the latest from git.git (commit 1eaabe34fc6f486367a176207420378f587d3b48, tagged v2.16.0-rc0).

-zefram