gitweb: Split body of git_search into subroutines

gitweb: Split body of git_search into subroutines

Create separate subroutines for handling each of aspects of searching
the repository:

 * git_search_message ('commit', 'author', 'committer')
 * git_search_changes ('pickaxe')
 * git_search_content_of_files ('grep')

Almost pure code movement (and unindent), which you can check e.g. via

  $ git blame -w --date=short -C -C HEAD^..HEAD -- gitweb/gitweb.perl |
    grep -C 3 -e '^[^^]' | less -S

No functional changes intended.

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