gitweb.cgi: Teach "a=blob" action to know the blob/file mime type

gitweb.cgi: Teach "a=blob" action to know the blob/file mime type

Now action "blob" knows the file type: if the file type is
not "text/*" then action "blob" defaults to "blob_plain",
i.e. the file is downloaded raw for the browser to interpret.
If the file type is "text/*", then "blob" defaults to the
current "cat -n"-like output, from which you can click
"plain", to get the "blob_plain" output.

Signed-off-by: Luben Tuikov <redacted>
Signed-off-by: Junio C Hamano <redacted>