Trivial fix in the italian translation

This is the current output of `git branch -h`:

$ git branch -h
uso: git branch [<opzioni>] [-r | -a] [--merged] [--no-merged]
  oppure: git branch [<options>] [-f] [--recurse-submodules] <branch-name> [<start-point>]
  oppure: git branch [<options>] [-l] [<pattern>...]
  oppure: git branch [<opzioni>] [-r] (-d | -D) <nome branch>...
  oppure: git branch [<opzioni>] (-m | -M) [<vecchio branch>] <nuovo branch>
  oppure: git branch [<opzioni>] (-c | -C) [<vecchio branch>] <nuovo branch>
  oppure: git branch [<opzioni>] [-r | -a] [--points-at]
  oppure: git branch [<opzioni>] [-r | -a] [--format]
[...]

As you can see `uso:` (`usage:`) and `oppure:` (`or:`) are misaligned. As commented in the .po the colon of `oppure:` should be aligned with the one of `usage:`. In the following patch I've substituted `oppure` with `o` (they're interchangable, and IMHO in the terminal every column counts) and I've aligned the colons in the two strings.