gitweb: Allow PNG, GIF, JPEG images to be displayed in "blob" view

gitweb: Allow PNG, GIF, JPEG images to be displayed in "blob" view

Allow images in one of web formats (PNG, GIF, JPEG) - actually files
with mimetype of image/png, image/git, image/jpeg - to be displayed in
"blob" view using <img /> element, instead of using "blob_plain" view
for them, like for all other files except also text/* mimetype files.

This makes possible to easily go to file history, to HEAD version of
the file, to appropriate commit etc; all of those are not available
in "blob_plain" (raw) view.

Only text files can have "blame" view link in the formats part of
navbar.

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