Handle hunks with removals and multiple hunks in a file properly

[nickolay]

This is a fix for #6.

Github doesn't display rebased commits in an order that makes sense, so here's they are in reverse-chronological order:

• ed40bb5 Remove manual test, as requested
• Fix 2 (deal with multiple hunks in a file)
  • 83a2d40 Fix handling of multiple hunks in one file
• Fix 1 (off-by-one error with hunks that remove content)
  • 4bdfd63 Make it clearer why the previous fix was correct
  • 6cf3e46 Properly create fixup commits for hunks with line removals
• Misc changes, separated to make the diffs for the fixes more readable:
  • c565a83 Move the "skipped non-modified hunk" check to the outer loop
  • 41acdb1 Introduce Hunk::header()
  • cfd1b8e End "fixup!" commit messages with newline
  • 88b59c4 Update Cargo.lock with the autogenerated message added in newer cargo

[tummychow]

much appreciated, thank you

[nickolay]

Thank you the quick review. And huge thanks for making this tool!