mingw: initialize HOME on startup

HOME initialization was historically duplicated in many different places,
including /etc/profile, launch scripts such as git-bash.vbs and gitk.cmd,
and (although slightly broken) in the git-wrapper.

Even unrelated projects such as GitExtensions and TortoiseGit need to
implement the same logic to be able to call git directly.

Initialize HOME in git's own startup code so that we can eventually retire
all the duplicate initialization code.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -2299,6 +2299,30 @@ static void setup_windows_environment(void)
 	/* simulate TERM to enable auto-color (see color.c) */
 	if (!getenv("TERM"))
 		setenv("TERM", "cygwin", 1);
+
+	/* calculate HOME if not set */
+	if (!getenv("HOME")) {
+		/*
+		 * try $HOMEDRIVE$HOMEPATH - the home share may be a network
+		 * location, thus also check if the path exists (i.e. is not
+		 * disconnected)
+		 */
+		if ((tmp = getenv("HOMEDRIVE"))) {
+			struct strbuf buf = STRBUF_INIT;
+			strbuf_addstr(&buf, tmp);
+			if ((tmp = getenv("HOMEPATH"))) {
+				strbuf_addstr(&buf, tmp);
+				if (is_directory(buf.buf))
+					setenv("HOME", buf.buf, 1);
+				else
+					tmp = NULL; /* use $USERPROFILE */
+			}
+			strbuf_release(&buf);
+		}
+		/* use $USERPROFILE if the home share is not available */
+		if (!tmp && (tmp = getenv("USERPROFILE")))
+			setenv("HOME", tmp, 1);
+	}
 }
 
 /*