[PATCH 0/3 v2] diff --stat: use the real terminal width

Hi, this is version 2. Changes:

- style fixes
- some tests for git-format-patch added
- patches 3 and 4 squashed together, since they touch the same lines
- graph width is limited to 40 columns, even if there's more space
- patch descriptions extended and cleared up

Description (modified):

this is a patch series to make 'git diff --stat' use full terminal width instead of hard-coded 80 columns.

This is quite useful when working on projects with nested directory
structure, e.g. Java:
 .../{ => workspace/tasks}/GetTaskResultAction.java |   10 +-
 .../tasks}/RemoveAllAbortedTasksAction.java        |    7 +-
 .../tasks}/RemoveAllFailedTasksAction.java         |    7 +-
is changed to display full paths if the terminal window is wide
enough.

Git usually uses the full terminal width automatically, so it should do so with --stat too.

The "big" functional change in the patch series is s/80/term_columns()/ in show_stats(). The partitioning of available columns is changed to dedicate more space to file names and the number of columns used for +- graph is limited.