Introduce `git_sysdir_expand_global_file`

Provide a mechanism for callers to expand the full path of a file in the
global configuration directory (that is to say, the home directory) even
if the file doesn't necessarily exist.  This lets callers use their own
logic for building paths separate from handling file existence.

src/sysdir.c

@@ -275,3 +275,14 @@ int git_sysdir_find_template_dir(git_buf *path)
 		path, NULL, GIT_SYSDIR_TEMPLATE, "template");
 }
 
+int git_sysdir_expand_global_file(git_buf *path, const char *filename)
+{
+	int error;
+
+	if ((error = git_sysdir_find_global_file(path, NULL)) == 0) {
+		if (filename)
+			error = git_buf_joinpath(path, path->ptr, filename);
+	}
+
+	return error;
+}

src/sysdir.h

@@ -55,6 +55,18 @@ extern int git_sysdir_find_programdata_file(git_buf *path, const char *filename)
  */
 extern int git_sysdir_find_template_dir(git_buf *path);
 
+/**
+ * Expand the name of a "global" file (i.e. one in a user's home
+ * directory).  Unlike `find_global_file` (above), this makes no
+ * attempt to check for the existence of the file, and is useful if
+ * you want the full path regardless of existence.
+ *
+ * @param path buffer to write the full path into
+ * @param filename name of file in the home directory
+ * @return 0 on success or -1 on error
+ */
+extern int git_sysdir_expand_global_file(git_buf *path, const char *filename);
+
 typedef enum {
 	GIT_SYSDIR_SYSTEM = 0,
 	GIT_SYSDIR_GLOBAL = 1,