Support git config [include] section with absolute path(s)

Support git config [include] section with absolute path(s)

As per [1], but limited to absolute paths indeed. No support yet for
tilde or $HOME expansion. Support for the --[no-]includes options
([1]) is not part of this commit scope either, but those options'
defaults are in effect as described in [1].

[1] https://git-scm.com/docs/git-config

Included path can be a config file that includes other path-s in turn.
An exception is thrown if too many recursions (circular includes)
happen because of ill-specified config files.

Change-Id: I700bd7b7e1625eb7de0180f220c707d8e7b0930b
Signed-off-by: Marco Miller <marco.miller@ericsson.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>