reset $pathspec: exit with code 0 if successful

reset $pathspec: exit with code 0 if successful

"git reset $pathspec" currently exits with a non-zero exit code if the
worktree is dirty after resetting, which is inconsistent with reset
without pathspec, and it makes it harder to know whether the command
really failed. Change it to exit with code 0 regardless of whether the
worktree is dirty so that non-zero indicates an error.

This makes the 4 "disambiguation" test cases in t7102 clearer since
they all used to "fail", 3 of which "failed" due to changes in the
work tree. Now only the ambiguous one fails.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>