WIP: set instaweb.httpd = mongoose globally

WIP because I'd like to support both old-style and new-style configs by
having mongoose and mongoose3 as possible httpd values and we would then
use mongoose3 in /etc/gitconfig.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
diff --git a/etc/gitconfig b/etc/gitconfig
index 320b95a..8cd4d78 100644
--- a/etc/gitconfig
+++ b/etc/gitconfig
@@ -19,3 +19,5 @@
 	textconv = astextplain
 [rebase]
 	autosquash = true
+[instaweb]
+	httpd = mongoose