Extract filepair_cb from ui-patch.c

Move filepair_cb() from ui-patch.c to ui-shared.c and rename it to
filepair_cb_raw(). This callback will be used in ui-diff.c in a
follow-up patch.

Note that it is not straightforward to extract filepair_cb() from
ui-diff.c which is why it is not done here as well.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
Allow for creating raw diffs with cgit_print_diff()

This adds a parameter to cgit_print_diff() to create raw diffs, using
the same format as `git diff <commit>`.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
cmd.c: Add a "rawdiff" command

This can be used to generate raw diffs between arbitrary revisions using
something like

     /rawdiff/?id=v0.9&id2=v0.9.1

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
ui-diff: Check the return value of get_sha1()

Sync with what we do everywhere else and check the return value of
get_sha1() instead of calling sha1_object_info() to validate the object.
Note that we later call lookup_commit_reference(), which checks that
both SHA1 values refer to commits, anyway.

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