Rollup merge of rust-lang#35085 - dns2utf8:git_clone_depth, r=alexcrichton

Reduce git clone --depth from 50 (default) to 1

This should speedup the builds a little.

Author: sanxiyn

.travis.yml

@@ -7,6 +7,7 @@ services:
 # our configure script, so disable auto submodule management.
 git:
   submodules: false
+  depth: 1
 
 before_install:
   - docker build -t rust -f src/etc/Dockerfile src/etc