sysdir: provide actual home directory

Provide a mechanism to look up the user's home directory, using the same
mechanism that we use for locating the global configuration path (a
fancy name for saying "the home directory").

SSH known hosts lookups now use this, instead of simply looking at the
HOME environment variable, to support Windows-style home directory
lookups in `HOME`, `HOMEPATH`, or `USERPROFILE`.

src/libgit2/sysdir.c

@@ -321,7 +321,7 @@ out:
 }
 #endif
 
-static int git_sysdir_guess_global_dirs(git_str *out)
+static int git_sysdir_guess_home_dirs(git_str *out)
 {
 #ifdef GIT_WIN32
 	static const wchar_t *global_tmpls[4] = {
@@ -367,6 +367,11 @@ static int git_sysdir_guess_global_dirs(git_str *out)
 #endif
 }
 
+static int git_sysdir_guess_global_dirs(git_str *out)
+{
+	return git_sysdir_guess_home_dirs(out);
+}
+
 static int git_sysdir_guess_xdg_dirs(git_str *out)
 {
 #ifdef GIT_WIN32
@@ -434,6 +439,7 @@ static struct git_sysdir__dir git_sysdir__dirs[] = {
 	{ GIT_STR_INIT, git_sysdir_guess_xdg_dirs },
 	{ GIT_STR_INIT, git_sysdir_guess_programdata_dirs },
 	{ GIT_STR_INIT, git_sysdir_guess_template_dirs },
+	{ GIT_STR_INIT, git_sysdir_guess_home_dirs }
 };
 
 static void git_sysdir_global_shutdown(void)
@@ -613,6 +619,12 @@ int git_sysdir_find_template_dir(git_str *path)
 		path, NULL, GIT_SYSDIR_TEMPLATE, "template");
 }
 
+int git_sysdir_find_homedir(git_str *path)
+{
+	return git_sysdir_find_in_dirlist(
+		path, NULL, GIT_SYSDIR_HOME, "home directory");
+}
+
 int git_sysdir_expand_global_file(git_str *path, const char *filename)
 {
 	int error;
@@ -624,3 +636,15 @@ int git_sysdir_expand_global_file(git_str *path, const char *filename)
 
 	return error;
 }
+
+int git_sysdir_expand_homedir_file(git_str *path, const char *filename)
+{
+	int error;
+
+	if ((error = git_sysdir_find_homedir(path)) == 0) {
+		if (filename)
+			error = git_str_joinpath(path, path->ptr, filename);
+	}
+
+	return error;
+}

src/libgit2/sysdir.h

@@ -57,10 +57,22 @@ extern int git_sysdir_find_programdata_file(git_str *path, const char *filename)
 extern int git_sysdir_find_template_dir(git_str *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.
+ * Find the home directory. On Windows, this will look at the `HOME`,
+ * `HOMEPATH`, and `USERPROFILE` environment variables (in that order)
+ * and return the first path that is set and exists. On other systems,
+ * this will simply return the contents of the `HOME` environment variable.
+ *
+ * @param path buffer to write the full path into
+ * @return 0 if found, GIT_ENOTFOUND if not found, or -1 on other OS error
+ */
+extern int git_sysdir_find_homedir(git_str *path);
+
+/**
+ * Expand the name of a "global" file -- by default inside the user's
+ * home directory, but can be overridden by the user configuration.
+ * 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
@@ -68,13 +80,25 @@ extern int git_sysdir_find_template_dir(git_str *path);
  */
 extern int git_sysdir_expand_global_file(git_str *path, const char *filename);
 
+/**
+ * Expand the name of a file in the user's home directory. This
+ * function 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_homedir_file(git_str *path, const char *filename);
+
 typedef enum {
-	GIT_SYSDIR_SYSTEM = 0,
-	GIT_SYSDIR_GLOBAL = 1,
-	GIT_SYSDIR_XDG    = 2,
+	GIT_SYSDIR_SYSTEM      = 0,
+	GIT_SYSDIR_GLOBAL      = 1,
+	GIT_SYSDIR_XDG         = 2,
 	GIT_SYSDIR_PROGRAMDATA = 3,
-	GIT_SYSDIR_TEMPLATE = 4,
-	GIT_SYSDIR__MAX   = 5
+	GIT_SYSDIR_TEMPLATE    = 4,
+	GIT_SYSDIR_HOME        = 5,
+	GIT_SYSDIR__MAX        = 6
 } git_sysdir_t;
 
 /**