Support %CURRENT% placeholder for Git branches

When run from a Git repository, cabal-meta will replace %CURRENT%
placeholders in sources.txt with the current Git branch name

Example:

When the current Git branch is 'new-ghc', the line in sources.txt

git://github.com/foo/pkg %CURRENT%

will be replaced with

git://github.com/foo/pkg new-ghc

When no Git repository is available in the current directory, no
replacement is performed.
Fix compilation