t5620: prepare branched repo for revision tests

t5620: prepare branched repo for revision tests

Prepare the test infrastructure for upcoming changes that teach 'git
backfill' to accept revision arguments and pathspecs.

Add test_tick before each commit in the setup loop so that commit dates
are deterministic. This enables reliable testing with '--since'.

Rename the 'd/e/' directory to 'd/f/' so that the prefix 'd/f' is
ambiguous with the files 'd/file.*.txt'. This exercises the subtlety
in prefix pathspec matching that will be added in a later commit.

Create a branched version of the test repository (src-revs) with:
 - A 'side' branch merged into main, adding s/file.{1,2}.txt with
   two versions (4 new blobs, 52 total from main HEAD).
 - An unmerged 'other' branch adding o/file.{1,2}.txt (2 more blobs,
   54 total reachable from --all).

This structure makes --all, --first-parent, and --since produce
meaningfully different results when used with 'git backfill'.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>