Revert "Add new @ shortcut for HEAD"

This reverts commit cdfd94837b, as it
does not just apply to "@" (and forms with modifiers like @{u}
applied to it), but also affects e.g. "refs/heads/@/foo", which it
shouldn't.

The basic idea of giving a short-hand might be good, and the topic
can be retried later, but let's revert to avoid affecting existing
use cases for now for the upcoming release.

Documentation/git-check-ref-format.txt

@@ -54,8 +54,6 @@ Git imposes the following rules on how references are named:
 
 . They cannot contain a sequence `@{`.
 
-. They cannot be the single character `@`.
-
 . They cannot contain a `\`.
 
 These rules make it easy for shell script based tools to parse

Documentation/revisions.txt

@@ -58,9 +58,6 @@ the '$GIT_DIR/refs' directory or from the '$GIT_DIR/packed-refs' file.
 While the ref name encoding is unspecified, UTF-8 is preferred as
 some output processing may assume ref names in UTF-8.
 
-'@'::
-  '@' alone is a shortcut for 'HEAD'.
-
 '<refname>@\{<date>\}', e.g. 'master@\{yesterday\}', 'HEAD@\{5 minutes ago\}'::
   A ref followed by the suffix '@' with a date specification
   enclosed in a brace

refs.c

@@ -72,10 +72,6 @@ int check_refname_format(const char *refname, int flags)
 {
 	int component_len, component_count = 0;
 
-	if (!strcmp(refname, "@"))
-		/* Refname is a single character '@'. */
-		return -1;
-
 	while (1) {
 		/* We are at the start of a path component. */
 		component_len = check_refname_component(refname, flags);

sha1_name.c

@@ -1004,17 +1004,6 @@ int get_sha1_mb(const char *name, unsigned char *sha1)
 	return st;
 }
 
-/* parse @something syntax, when 'something' is not {.*} */
-static int interpret_empty_at(const char *name, int namelen, int len, struct strbuf *buf)
-{
-	if (len || name[1] == '{')
-		return -1;
-
-	strbuf_reset(buf);
-	strbuf_add(buf, "HEAD", 4);
-	return 1;
-}
-
 static int reinterpret(const char *name, int namelen, int len, struct strbuf *buf)
 {
 	/* we have extra data, which might need further processing */
@@ -1077,15 +1066,9 @@ int interpret_branch_name(const char *name, struct strbuf *buf)
 	cp = strchr(name, '@');
 	if (!cp)
 		return -1;
-
-	len = interpret_empty_at(name, namelen, cp - name, buf);
-	if (len > 0)
-		return reinterpret(name, namelen, len, buf);
-
 	tmp_len = upstream_mark(cp, namelen - (cp - name));
 	if (!tmp_len)
 		return -1;
-
 	len = cp + tmp_len - name;
 	cp = xstrndup(name, cp - name);
 	upstream = branch_get(*cp ? cp : NULL);

t/t1508-at-combinations.sh

@@ -55,8 +55,6 @@ check "HEAD@{u}" ref refs/heads/upstream-branch
 check "@{u}@{1}" commit upstream-one
 check "@{-1}@{u}" ref refs/heads/master
 check "@{-1}@{u}@{1}" commit master-one
-check "@" commit new-two
-check "@@{u}" ref refs/heads/upstream-branch
 nonsense "@{u}@{-1}"
 nonsense "@{0}@{0}"
 nonsense "@{1}@{u}"