checkout: describe_detached_head: remove ellipsis after committish

checkout: describe_detached_head: remove ellipsis after committish

We do not want an ellipsis displayed following an (abbreviated) SHA-1
value.

The days when this was necessary to indicate the truncation to
lower-level Git commands and/or the user are bygone.

However, to ease the transition, the ellipsis will still be printed if
the user sets the environment variable GIT_PRINT_SHA1_ELLIPSIS to "yes".

Correct documentation with respect to what describe_detached_head prints
when GIT_PRINT_SHA1_ELLIPSIS is not set as indicated above.

Add tests for the old and new behaviour.

Signed-off-by: Ann T Ropea <redacted>
Signed-off-by: Junio C Hamano <redacted>