combine-diff: move changed-paths scanning logic into its own function

combine-diff: move changed-paths scanning logic into its own function

Move code for finding paths for which diff(commit,parent_i) is not-empty
for all parents to separate function - at present we have generic (and
slow) code for this job, which translates 1 n-parent problem to n
1-parent problems and then intersect results, and will be adding another
limited, but faster, paths scanning implementation in the next patch.

Signed-off-by: Kirill Smelkov <redacted>
Signed-off-by: Junio C Hamano <redacted>