Fix JGit CLI to follow native git's interpretation of http_proxy...

Fix JGit CLI to follow native git's interpretation of http_proxy...

Native git (as many other tools) interprets the environment variables
http_proxy, HTTP_PROXY, ... in a specific way. "http_proxy" has to be
lowercase while "https_proxy" can be lowercase or uppercase (means:
"HTTPS_PROXY"). Lowercase has precedence. This can be looked up in
"ENVIRONMENT" section of [1]. Teach JGit CLI to behave similar.

Additionally teach JGit not to interpret the environment variables if
the java process was explicitly started with the system properties
telling JVM which proxy to use. A call like "http_proxy=proxy1 java
-Dhttp.proxyHost=proxy2 ..." should use proxy2 as proxy.

[1] https://curl.haxx.se/docs/manpage.html

Change-Id: I2ad78f209792bf8f1285cf2f8ada8ae0c28f8e5a