t4211: document why pickaxe commit-level filtering remains broken

Add a comment above the test_expect_failure tests explaining that
show_log() prints the commit header before diffcore_std() runs
pickaxe, so commits cannot be suppressed even when no diff pairs
survive filtering.  Fixing this would require deferring show_log()
until after diffcore_std(), which is a larger restructuring of the
log-tree output pipeline.

Signed-off-by: Michael Montalbo <mmontalbo@gmail.com>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mmontalbo

t/t4211-line-log.sh

@@ -494,6 +494,12 @@ test_expect_success '-L --find-object does not crash with merge and rename' '
 		--find-object=$(git rev-parse HEAD:file) >actual
 '
 
+# Commit-level filtering with pickaxe does not yet work for -L.
+# show_log() prints the commit header before diffcore_std() runs
+# pickaxe, so commits cannot be suppressed even when no diff pairs
+# survive filtering.  Fixing this would require deferring show_log()
+# until after diffcore_std(), which is a larger restructuring of the
+# log-tree output pipeline.
 test_expect_failure '-L -G should filter commits by pattern' '
 	git log --format="%s" --no-patch -L 1,1:file -G "nomatch" >actual &&
 	test_must_be_empty actual