Merge branch 'ho/gitweb-squelch-undef-warning'

Asking gitweb for a nonexistent commit left a warning in the server log. Somebody may want to follow this up with a new test, perhaps? IIRC, we do test that no Perl warnings are given to the server log, so this should have been caught if our test coverage were good. * ho/gitweb-squelch-undef-warning: gitweb: squelch "uninitialized value" warning