Merge branch 'rs/combine-diff-zero-context-at-the-beginning'

Merge branch 'rs/combine-diff-zero-context-at-the-beginning'

Fixes an age old corner case bug in combine diff (only triggered with -U0
and the hunk at the beginning of the file needs to be shown).

By René Scharfe
* rs/combine-diff-zero-context-at-the-beginning:
  combine-diff: fix loop index underflow