replace-refs: Fix "invalid replace ref" warning during decoration

The enactment of replace refs in
replace-object.c#register_replace_ref() explicitly uses the last
portion of the ref "path", the substring after the last slash, as the
object ID to be replaced. This means that replace refs can be
organized into different paths; you can separate replace refs created
for different purposes, like "refs/replace/2012-migration/*". This in
turn makes it practical to store prepared replace refs in different ref
paths on a git server, and have users "map" them, via specific
refspecs, into their local repos; different types of replacements can
be mapped into different sub-paths of refs/replace/.

The only way this didn't "work" is in the commit decoration process,
in log-tree.c#add_ref_decoration(), where different logic was used to
obtain the replaced object ID, removing the "refs/replace/" prefix
only. This inconsistent logic meant that more structured replace ref
paths caused a warning to be printed, and the "replaced" decoration to
be omitted.

Fix this decoration logic to use the same "last part of ref path"
logic, fixing spurious warnings (and missing decorations) for users
of more structured replace ref paths. Also add tests for nested replace
ref paths.

Author: TaoK

log-tree.c

@@ -163,10 +163,11 @@ static int add_ref_decoration(const char *refname, const char *referent UNUSED,
 
 	if (starts_with(refname, git_replace_ref_base)) {
 		struct object_id original_oid;
+		const char *slash = strrchr(refname, '/');
+		const char *hash = slash ? slash + 1 : refname;
 		if (!replace_refs_enabled(the_repository))
 			return 0;
-		if (get_oid_hex(refname + strlen(git_replace_ref_base),
-				&original_oid)) {
+		if (get_oid_hex(hash, &original_oid)) {
 			warning("invalid replace ref %s", refname);
 			return 0;
 		}

t/t4207-log-decoration-colors.sh

@@ -131,4 +131,21 @@ ${c_tag}tag: ${c_reset}${c_tag}A${c_reset}${c_commit})${c_reset} A
 	cmp_filtered_decorations
 '
 
+test_expect_success 'test replace decoration for nested replace path' '
+	test_when_finished remove_replace_refs &&
+
+	CURRENT_HASH=$(git rev-parse --verify HEAD) &&
+	git replace --graft HEAD HEAD~2 &&
+	git update-ref refs/tmp/tmpref refs/replace/$CURRENT_HASH &&
+	git update-ref -d refs/replace/$CURRENT_HASH &&
+	git update-ref refs/replace/nested-path/abc/$CURRENT_HASH refs/tmp/tmpref &&
+
+	git log --decorate -1 HEAD >actual &&
+	test_grep "replaced" actual &&
+
+	git --no-replace-objects log --decorate -1 HEAD >actual &&
+	test_grep ! "replaced" actual
+
+'
+
 test_done

t/t6050-replace.sh

@@ -546,4 +546,14 @@ test_expect_success '--convert-graft-file' '
 	test_grep "$EMPTY_BLOB $EMPTY_TREE" .git/info/grafts
 '
 
+test_expect_success 'replace ref in a nested path' '
+	git cat-file commit $HASH2 >actual &&
+	R=$(sed -e "s/A U/O/" actual | git hash-object -t commit --stdin -w) &&
+	git update-ref refs/replace/abc1/$HASH2 $R &&
+	git show $HASH2 >actual &&
+	test_grep "O Thor" actual &&
+	git --no-replace-objects show $HASH2 >actual &&
+	test_grep "A U Thor" actual
+'
+
 test_done