gitweb: Add charset info to "raw" output of 'text/plain' blobs

Earlier "blob_plain" view sent "charset=utf-8" only when gitweb
guessed the content type to be text by reading from it, and not when
the MIME type was obtained from /etc/mime.types, or when gitweb
couldn't guess mimetype and used $default_blob_plain_mimetype.

This fixes the bug by always add charset info from
$default_text_plain_charset (if it is defined) to "raw" (a=blob_plain)
output for 'text/plain' blobs.

Generating information for Content-Type: header got separated into
blob_contenttype() subroutine; adding charset info in a special case
was removed from blob_mimetype(), which now should return mimetype
only.

While at it cleanup code a bit: put subroutine parameter
initialization first, make error message more robust (when $file_name
is not defined) if more cryptic, remove unnecessary '"' around
variable ("$var" -> $var).

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