gitweb: Great subroutines renaming

gitweb: Great subroutines renaming

Rename some of subroutines to better reflect what they do.
Some renames were not performed because subroutine name
reflects hash key.

Subroutines name guideline:
* git_ prefix for subroutines related to git commands,
  git repository, or to gitweb actions
* git_get_ prefix for inner subroutines calling git command
  or reading some file in the repository and returning some output
* parse_ prefix for subroutines parsing some text (or reading and
  parsing some text) into hash or list
* format_ prefix for subroutines formatting, post-processing
  or generating some HTML/text fragment
* _get_ infix for subroutines which return result
* _print_ infix for subroutines which print fragment of output
* _body suffix for subroutines which outputs main part (body)
  of related action (usually table)
* _nav suffix for subroutines related to navigation bars
* _div suffix for subroutines returning or printing div element
* subroutine names should not be based on how the result is obtained,
  as this might change easily

Renames performed:
- git_get_referencing => format_ref_marker
- git_get_paging_nav => format_paging_nav
- git_read_head => git_get_head_hash
- git_read_hash => git_get_hash_by_ref
- git_read_description => git_get_project_description
- git_read_projects => git_get_projects_list
- read_info_ref => git_get_references
- git_read_refs => git_get_refs_list
- date_str => parse_date
- git_read_tag => parse_tag
- git_read_commit => parse_commit
- git_blob_plain_mimetype => blob_mimetype
- git_page_nav => git_print_page_nav
- git_header_div => git_print_header_div

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>