path.c: add git_common_path() and strbuf_git_common_path()

These are mostly convenient functions to reduce code duplication. Most
of the time, we should be able to get by with git_path() which handles
$GIT_COMMON_DIR internally. However there are a few cases where we need
to construct paths manually, for example some paths from a specific
worktree. These functions will enable that.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -799,11 +799,14 @@ extern void check_repository_format(void);
  */
 extern const char *mkpath(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
 extern const char *git_path(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
+extern const char *git_common_path(const char *fmt, ...) __attribute__((format (printf, 1, 2)));
 
 extern char *mksnpath(char *buf, size_t n, const char *fmt, ...)
 	__attribute__((format (printf, 3, 4)));
 extern void strbuf_git_path(struct strbuf *sb, const char *fmt, ...)
 	__attribute__((format (printf, 2, 3)));
+extern void strbuf_git_common_path(struct strbuf *sb, const char *fmt, ...)
+	__attribute__((format (printf, 2, 3)));
 extern char *git_path_buf(struct strbuf *buf, const char *fmt, ...)
 	__attribute__((format (printf, 2, 3)));
 extern void strbuf_git_path_submodule(struct strbuf *sb, const char *path,

path.c

@@ -503,6 +503,35 @@ void strbuf_git_path_submodule(struct strbuf *buf, const char *path,
 	va_end(args);
 }
 
+static void do_git_common_path(struct strbuf *buf,
+			       const char *fmt,
+			       va_list args)
+{
+	strbuf_addstr(buf, get_git_common_dir());
+	if (buf->len && !is_dir_sep(buf->buf[buf->len - 1]))
+		strbuf_addch(buf, '/');
+	strbuf_vaddf(buf, fmt, args);
+	strbuf_cleanup_path(buf);
+}
+
+const char *git_common_path(const char *fmt, ...)
+{
+	struct strbuf *pathname = get_pathname();
+	va_list args;
+	va_start(args, fmt);
+	do_git_common_path(pathname, fmt, args);
+	va_end(args);
+	return pathname->buf;
+}
+
+void strbuf_git_common_path(struct strbuf *sb, const char *fmt, ...)
+{
+	va_list args;
+	va_start(args, fmt);
+	do_git_common_path(sb, fmt, args);
+	va_end(args);
+}
+
 int validate_headref(const char *path)
 {
 	struct stat st;