ui-patch.c: Use log_tree_commit() to generate diffs

Instead of using our own formatting, use log_tree_commit() from Git to
create patches. This removes unnecessary duplicate code and also fixes a
bug with e-mail address formatting that existed in our own
implementation.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
Allow for creating patch series

This allows for specifying a revision range using the id2 parameter of
/patch/. The output that is produced is similar to

    $ git format-patch --stdout id2..id

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
ui-patch: Rename variables

Rename parameters and local variables to match those from ui-diff. Also,
convert a "char *" to "const char *".

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>