path.c: implement xdg_config_home()

The XDG base dir spec[1] specifies that configuration files be stored in
a subdirectory in $XDG_CONFIG_HOME. To construct such a configuration
file path, home_config_paths() can be used. However, home_config_paths()
combines distinct functionality:

1. Retrieve the home git config file path ~/.gitconfig

2. Construct the XDG config path of the file specified by `file`.

This function was introduced in commit 21cf322 ("read (but not write)
from $XDG_CONFIG_HOME/git/config file").  While the intention of the
function was to allow the home directory configuration file path and the
xdg directory configuration file path to be retrieved with one function
call, the hard-coding of the path ~/.gitconfig prevents it from being
used for other configuration files. Furthermore, retrieving a file path
relative to the user's home directory can be done with
expand_user_path(). Hence, it can be seen that home_config_paths()
introduces unnecessary complexity, especially if a user just wants to
retrieve the xdg config file path.

As such, implement a simpler function xdg_config_home() for constructing
the XDG base dir spec configuration file path. This function, together
with expand_user_path(), can replace all uses of home_config_paths().

[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-0.7.html

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -828,6 +828,13 @@ char *strip_path_suffix(const char *path, const char *suffix);
 int daemon_avoid_alias(const char *path);
 extern int is_ntfs_dotgit(const char *name);
 
+/**
+ * Return a newly allocated string with the evaluation of
+ * "$XDG_CONFIG_HOME/git/$filename" if $XDG_CONFIG_HOME is non-empty, otherwise
+ * "$HOME/.config/git/$filename". Return NULL upon error.
+ */
+extern char *xdg_config_home(const char *filename);
+
 /* object replacement */
 #define LOOKUP_REPLACE_OBJECT 1
 extern void *read_sha1_file_extended(const unsigned char *sha1, enum object_type *type, unsigned long *size, unsigned flag);

path.c

@@ -856,3 +856,18 @@ int is_ntfs_dotgit(const char *name)
 			len = -1;
 		}
 }
+
+char *xdg_config_home(const char *filename)
+{
+	const char *home, *config_home;
+
+	assert(filename);
+	config_home = getenv("XDG_CONFIG_HOME");
+	if (config_home && *config_home)
+		return mkpathdup("%s/git/%s", config_home, filename);
+
+	home = getenv("HOME");
+	if (home)
+		return mkpathdup("%s/.config/git/%s", home, filename);
+	return NULL;
+}