gitweb: Smarter snapshot names

gitweb: Smarter snapshot names

Teach gitweb how to produce nicer snapshot names by only using the
short hash id.  If clients make requests using a tree-ish that is not
a partial or full SHA-1 hash, then the short hash will also be appended
to whatever they asked for.  If clients request snapshot of a tag
(which means that $hash ('h') parameter has 'refs/tags/' prefix),
use only tag name.

Update tests cases in t9502-gitweb-standalone-parse-output.

Gitweb uses the following format for snapshot filenames:
  <sanitized project name>-<version info>.<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:
  <sanitized project name>-<version info>/
as compared to <sanitized project name>/ before (without version info).

Current rules for <version info>:
* if 'h' / $hash parameter is SHA-1 or shortened SHA-1, use SHA-1
  shortened to to 7 characters
* otherwise if 'h' / $hash parameter is tag name (it begins with
  'refs/tags/' prefix, use tag name (with 'refs/tags/' stripped
* otherwise if 'h' / $hash parameter starts with 'refs/heads/' prefix,
  strip this prefix, convert '/' into '.', and append shortened SHA-1
  after '-', i.e. use <sanitized hash>-<shortened sha1>

Signed-off-by: Mark Rada <marada@uwaterloo.ca>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>