Output paths relative to current working directory

[dandavison]

Fixes #552

TODO:

• Make things correct when git's --relative is in effect
• Add option to control this: Done: --relative-paths
• Add option to control padding width when this is active: Done: this is --diff-stat-align-width
• Would --max-relative-path-length be a better name? No.

Out of scope for this PR:

• Process all lines in the diff stat before emitting any of them so that the max path length can be determined.