cgit.c: Use "else" for mutually exclusive branches

When parsing command line arguments, no pair of command line options can
ever match simultaneously. Use "else if" blocks to reflect this. This
change improves both readability and speed.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>
cgit.c: Fix comment on bit mask hack

* Formatting and spelling fixes.

* A bit mask with the size of one byte only allows for storing 8 (not
  255!) different flags.

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