Fix parsing of repo info, for case when current dir starting from dot

In this case command
$(git rev-parse --git-dir --is-inside-work-tree --show-cdup --show-prefix)
will print some thing like this:
{code}
/tig/.git
true
../
.started-from-dot/
{/code}

And after this in read_repo_info() it will overwrite "opt_cdup", by
"--show-prefix" value, and for example $(tig blame ../io.h) from
".started-from-dot" directory will fail, because of wrong opt_cdup.