gitweb: Fix "heads" view when there is no current branch

gitweb: Fix "heads" view when there is no current branch

In a repository whose HEAD points to an unborn branch with no commits,
"heads" view and "summary" view (which shows what is shown in "heads"
view) compared the object names of commits at the tip of branches with the
output from "git rev-parse HEAD", which caused comparison of a string with
undef and resulted in a warning in the server log.

This can happen if non-bare repository (with default 'master' branch)
is updated not via committing but by other means like push to it, or
Gerrit.  It can happen also just after running "git checkout --orphan
<new branch>" but before creating any new commit on this branch.

Rewrite the comparison so that it also works when $head points at nothing;
in such a case, no branch can be "the current branch", add a test for it.
While at it, rename local variable $head to $head_at, as it points to
current commit rather than current branch name (HEAD contents).

The code still incorrectly shows all branches that point at the same
commit as what HEAD points as "the current branch", even when HEAD is
detached. Fixing this bug is outside the scope of this patch.

Reported-by: Rajesh Boyapati
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>