gitweb: Create special from-file/to-file header for combined diff

gitweb: Create special from-file/to-file header for combined diff

Instead of using default, diff(1) like from-file/to-file header for
combined diff (for a merge commit), which looks like:

  --- a/git-gui/git-gui.sh
  +++ b/_git-gui/git-gui.sh_

(where _link_ denotes [hidden] hyperlink), create from-file(n)/to-file
header, using "--- <n>/_<filename>_" for each of parents, e.g.:

  --- 1/_git-gui/git-gui.sh_
  --- 2/_git-gui.sh_
  +++ b/_git-gui/git-gui.sh_

Test it on one of merge commits involving rename, e.g.
  95f97567c1887d77f3a46b42d8622c76414d964d (rename at top)
  5bac4a671907604b5fb4e24ff682d5b0e8431931 (file from one branch)

This is mainly meant to easier see renames in a merge commit.

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