gitweb: Fix bug in "blobdiff" view for split (e.g. file to symlink) patches

gitweb: Fix bug in "blobdiff" view for split (e.g. file to symlink) patches

git_patchset_body needs patch generated with --full-index option to
detect split patches, meaning two patches which corresponds to single
difftree (raw diff) entry.  An example of such situation is changing
type (mode) of a file, e.g. from plain file to symbolic link.

Add, in git_blobdiff, --full-index option to patch generating git diff
invocation, for the 'html' format output ("blobdiff" view).

"blobdiff_plain" still uses shortened sha1 in the extended git diff
header "index <hash>..<hash>[ <mode>]" line.

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