gitweb: Default to $hash_base or HEAD for $hash in "commit" and "commitdiff"

Set $hash parameter to $hash_base || "HEAD" if it is not set (if it is
not true to be more exact). This allows [hand-edited] URLs with 'action'
"commit" or "commitdiff" but without 'hash' parameter.

If there is 'h' (hash) parameter provided, then gitweb tries
to use this. HEAD is used _only_ if nether hash, nor hash_base
are provided, i.e. for URL like below
  URL?p=project.git;a=commit
i.e. without neither 'h' nor 'hb'.

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