branch: force-copy a branch to itself via @{-1} is a no-op

Since 52d59cc645 (branch: add a --copy (-c) option to go with --move
(-m), 2017-06-18) we can copy a branch to make a new branch with the
'-c' (copy) option or to overwrite an existing branch using the '-C'
(force copy) option.  A no-op possibility is considered when we are
asked to copy a branch to itself, to follow the same no-op introduced
for the rename (-M) operation in 3f59481e33 (branch: allow a no-op
"branch -M <current-branch> HEAD", 2011-11-25).  To check for this, in
52d59cc645 we compared the branch names provided by the user, source
(HEAD if omitted) and destination, and a match is considered as this
no-op.

Since ae5a6c3684 (checkout: implement "@{-N}" shortcut name for N-th
last branch, 2009-01-17) a branch can be specified using shortcuts like
@{-1}.  This allows this usage:

	$ git checkout -b test
	$ git checkout -
	$ git branch -C test test  # no-op
	$ git branch -C test @{-1} # oops
	$ git branch -C @{-1} test # oops

As we are using the branch name provided by the user to do the
comparison, if one of the branches is provided using a shortcut we are
not going to have a match and a call to git_config_copy_section() will
happen.  This will make a duplicate of the configuration for that
branch, and with this progression the second call will produce four
copies of the configuration, and so on.

Let's use the interpreted branch name instead for this comparison.

The rename operation is not affected.

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

builtin/branch.c

@@ -584,13 +584,13 @@ static void copy_or_rename_branch(const char *oldname, const char *newname, int
 	strbuf_release(&logmsg);
 
 	strbuf_addf(&oldsection, "branch.%s", interpreted_oldname);
-	strbuf_release(&oldref);
 	strbuf_addf(&newsection, "branch.%s", interpreted_newname);
-	strbuf_release(&newref);
 	if (!copy && git_config_rename_section(oldsection.buf, newsection.buf) < 0)
 		die(_("Branch is renamed, but update of config-file failed"));
-	if (copy && strcmp(oldname, newname) && git_config_copy_section(oldsection.buf, newsection.buf) < 0)
+	if (copy && strcmp(interpreted_oldname, interpreted_newname) && git_config_copy_section(oldsection.buf, newsection.buf) < 0)
 		die(_("Branch is copied, but update of config-file failed"));
+	strbuf_release(&oldref);
+	strbuf_release(&newref);
 	strbuf_release(&oldsection);
 	strbuf_release(&newsection);
 }

t/t3204-branch-name-interpretation.sh

@@ -57,6 +57,16 @@ test_expect_success 'create branch with pseudo-qualified name' '
 	expect_branch refs/heads/refs/heads/qualified two
 '
 
+test_expect_success 'force-copy a branch to itself via @{-1} is no-op' '
+	git branch -t copiable main &&
+	git checkout copiable &&
+	git checkout - &&
+	git branch -C @{-1} copiable &&
+	git config --get-all branch.copiable.merge >actual &&
+	echo refs/heads/main >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'delete branch via @{-1}' '
 	git branch previous-del &&