gitweb: Document current snapshot rules via new tests

gitweb: Document current snapshot rules via new tests

Add t9502-gitweb-standalone-parse-output test script, which runs
gitweb as a CGI script from the commandline and checks that it
produces the correct output.

Currently this test script contains only tests of snapshot naming
(proposed name of snapshot file) and snapshot prefix (prefix of files
in the archive / snapshot).  It defines and uses 'tar' snapshot
format, without compression, for easy checking of snapshot prefix.
Testing is done using check_snapshot function.

Gitweb uses the following format for snapshot filenames:
  <sanitized project name>-<hash parameter><snapshot suffix>
where <sanitized project name> is project name with '.git' or '/.git'
suffix stripped, unless '.git' is the whole project name.  For
snapshot prefix it uses simply:
  <sanitized project name>/

Disadvantages of current snapshot rules:
* There exists convention that <basename>.<suffix> archive unpacks to
  <basename>/ directory (<basename>/ is prefix of archive).  Gitweb
  does not respect it
* Snapshot links generated by gitweb use full SHA-1 id as a value of
  'h' / $hash parameter.  With current rules it leads to long file
  names like e.g. repo-1005c80cc11c531d327b12195027cbbb4ff9e3cb.tgz
* For handcrafted URLs, where 'h' / $hash parameter is a symbolic
  'volatile' revision name such as "HEAD" or "next" snapshot name
  doesn't tell us what exact version it was created from
* Proposed filename in Content-Disposition header should not contain
  any directory path information, which means that it should not
  contain '/' (see RFC2183)... which means that snapshot naming is
  broken for $hash being e.g. hirearchical branch name such as
  'xx/test'

This would be improved in next commit.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>