test-pathencode: more aggressively check for python < 2.6

diff: move index header generation to patch In an upcoming patch, we will add index information to all git diffs, not only binary diffs, so this code needs to be moved to a more appropriate place. Also, since this information is used for patch headers, it makes more sense to be in the patch module, along with other patch-related metadata.

patch: make _addmodehdr a function under trydiff addmodehdr is a header helper, same as diffline, so it doesn't need to be a top-level function and can be nested under trydiff. In upcoming patches we will generalize this approach for all headers.

diff: rewrite diffline Make diffline more readable, using strings with placeholders rather than appending to a list from many ifs that makes difficult to understand the actual output format.