expand_user_path: expand ~ to $HOME, not to the actual homedir.

In 395de250d (Expand ~ and ~user in core.excludesfile, commit.template),
we introduced the mechanism. But expanding ~ using getpw is not what
people overriding $HOME would usually expect. In particular, git looks
for the user's .gitconfig using $HOME, so it's better to be consistent.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -380,9 +380,9 @@ Common unit suffixes of 'k', 'm', or 'g' are supported.
 core.excludesfile::
 	In addition to '.gitignore' (per-directory) and
 	'.git/info/exclude', git looks into this file for patterns
-	of files which are not meant to be tracked.  "~/" and "~user/"
-	are expanded to the specified user's home directory.  See
-	linkgit:gitignore[5].
+	of files which are not meant to be tracked.  "~/" is expanded
+	to the value of `$HOME` and "~user/" to the specified user's
+	home directory.  See linkgit:gitignore[5].
 
 core.editor::
 	Commands such as `commit` and `tag` that lets you edit
@@ -667,7 +667,8 @@ color.ui::
 
 commit.template::
 	Specify a file to use as the template for new commit messages.
-	"~/" and "~user/" are expanded to the specified user's home directory.
+	"~/" is expanded to the value of `$HOME` and "~user/" to the
+	specified user's home directory.
 
 diff.autorefreshindex::
 	When using 'git-diff' to compare with work tree

path.c

@@ -235,10 +235,15 @@ char *expand_user_path(const char *path)
 	if (path[0] == '~') {
 		const char *username = path + 1;
 		size_t username_len = first_slash - username;
-		struct passwd *pw = getpw_str(username, username_len);
-		if (!pw)
-			goto return_null;
-		strbuf_add(&user_path, pw->pw_dir, strlen(pw->pw_dir));
+		if (username_len == 0) {
+			const char *home = getenv("HOME");
+			strbuf_add(&user_path, home, strlen(home));
+		} else {
+			struct passwd *pw = getpw_str(username, username_len);
+			if (!pw)
+				goto return_null;
+			strbuf_add(&user_path, pw->pw_dir, strlen(pw->pw_dir));
+		}
 		to_copy = first_slash;
 	}
 	strbuf_add(&user_path, to_copy, strlen(to_copy));