get_main_worktree(): allow it to be called in the Git directory

get_main_worktree(): allow it to be called in the Git directory

When called in the Git directory of a non-bare repository, this function
would not return the directory of the main worktree, but of the Git
directory instead.

The reason: when the Git directory is the current working directory, the
absolute path of the common directory will be reported with a trailing
`/.git/.`, which the code of `get_main_worktree()` does not handle
correctly.

Let's fix this.

Helped-by: Johannes Schindelin <redacted>
Signed-off-by: Hariom Verma <redacted>
Signed-off-by: Junio C Hamano <redacted>