rev-parse --is-bare-repository incorrect when work tree is set

[Sam Bazley <sambazley@xxxxxxxxxxxx>]

rev-parse --is-bare-repository incorrectly outputs "false" when testing
a bare repository with the work tree set.

To reproduce:

$ git init --bare testbare
$ git --git-dir=testbare rev-parse --is-bare-repository
true
$ git --git-dir=testbare --work-tree=something rev-parse \
--is-bare-repository
false
$ GIT_WORK_TREE=something git --git-dir=testbare rev-parse \
--is-bare-repository
false