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

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) has been
fixed.

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

diff --cc combine-diff.c
Simple merge