core.prefersymlinkrefs: use symlinks for .git/HEAD

When inspecting a project whose build infrastructure used to
assume that .git/HEAD is a symlink ref, core.prefersymlinkrefs
in the config file of such a project would help to bisect its
history.

Signed-off-by: Junio C Hamano <junkio@cox.net>
(cherry picked from 9f0bb90d16 commit)

Documentation/config.txt

@@ -64,9 +64,11 @@ core.ignoreStat::
 	slow, such as Microsoft Windows.  See gitlink:git-update-index[1].
 	False by default.
 
-core.onlyUseSymrefs::
-	Always use the "symref" format instead of symbolic links for HEAD
-	and other symbolic reference files. True by default.
+core.preferSymlinkRefs::
+	Instead of the default "symref" format for HEAD
+	and other symbolic reference files, use symbolic links.
+	This is sometimes needed to work with old scripts that
+	expect HEAD to be a symbolic link.
 
 core.repositoryFormatVersion::
 	Internal variable identifying the repository format and layout

Makefile

@@ -28,8 +28,8 @@ all:
 #
 # Define NO_SETENV if you don't have setenv in the C library.
 #
-# Define USE_SYMLINK_HEAD if you want .git/HEAD to be a symbolic link.
-# Don't enable it on Windows.
+# Define NO_SYMLINK_HEAD if you never want .git/HEAD to be a symbolic link.
+# Enable it on Windows.  By default, symrefs are still used.
 #
 # Define PPC_SHA1 environment variable when running make to make use of
 # a bundled SHA1 routine optimized for PowerPC.
@@ -260,6 +260,7 @@ ifeq ($(uname_O),Cygwin)
 	NO_D_TYPE_IN_DIRENT = YesPlease
 	NO_D_INO_IN_DIRENT = YesPlease
 	NO_STRCASESTR = YesPlease
+	NO_SYMLINK_HEAD = YesPlease
 	NEEDS_LIBICONV = YesPlease
 	# There are conflicting reports about this.
 	# On some boxes NO_MMAP is needed, and not so elsewhere.
@@ -383,6 +384,9 @@ endif
 ifdef NO_D_INO_IN_DIRENT
 	ALL_CFLAGS += -DNO_D_INO_IN_DIRENT
 endif
+ifdef NO_SYMLINK_HEAD
+	ALL_CFLAGS += -DNO_SYMLINK_HEAD
+endif
 ifdef NO_STRCASESTR
 	COMPAT_CFLAGS += -DNO_STRCASESTR
 	COMPAT_OBJS += compat/strcasestr.o

cache.h

@@ -168,7 +168,7 @@ extern void rollback_index_file(struct cache_file *);
 /* Environment bits from configuration mechanism */
 extern int trust_executable_bit;
 extern int assume_unchanged;
-extern int only_use_symrefs;
+extern int prefer_symlink_refs;
 extern int warn_ambiguous_refs;
 extern int diff_rename_limit_default;
 extern int shared_repository;

config.c

@@ -227,8 +227,8 @@ int git_default_config(const char *var, const char *value)
 		return 0;
 	}
 
-	if (!strcmp(var, "core.symrefsonly")) {
-		only_use_symrefs = git_config_bool(var, value);
+	if (!strcmp(var, "core.prefersymlinkrefs")) {
+		prefer_symlink_refs = git_config_bool(var, value);
 		return 0;
 	}
 

environment.c

@@ -13,7 +13,7 @@ char git_default_email[MAX_GITNAME];
 char git_default_name[MAX_GITNAME];
 int trust_executable_bit = 1;
 int assume_unchanged = 0;
-int only_use_symrefs = 0;
+int prefer_symlink_refs = 0;
 int warn_ambiguous_refs = 1;
 int repository_format_version = 0;
 char git_commit_encoding[MAX_ENCODING_LENGTH] = "utf-8";

refs.c

@@ -76,8 +76,8 @@ int create_symref(const char *git_HEAD, const char *refs_heads_master)
 	char ref[1000];
 	int fd, len, written;
 
-#ifdef USE_SYMLINK_HEAD
-	if (!only_use_symrefs) {
+#ifndef NO_SYMLINK_HEAD
+	if (prefer_symlink_refs) {
 		unlink(git_HEAD);
 		if (!symlink(refs_heads_master, git_HEAD))
 			return 0;