Fix dev server configuration

The config option gitiles.basePath was overloaded in [1]
which broke the default behavior of gitiles in dev mode. It used to scan
the path configured via gitiles.basePath to find git repositories.

With [1] it now sets the servlet context path for gitiles to the value
of the same option which doesn't work since the dev mode by default sets
gitiles.basePath to the current working directory which doesn't work
as a servlet context path which results in gitiles returning
"404 Not Found" instead of showing the list of repositories found under
basePath.

Fix this by renaming the option introduced in [1] to
gitiles.contextPath and adapt the documentation accordingly.

[1] I1da95745d1bc8eaa7d5b204a942bcee0b6a027a8

Change-Id: I0f844eaaaadbfd21161e04b915b541a534bbb4a6