Move code interpreting path relative to exec-dir to new function system_path()
Expanding system paths relative to git_exec_path can be used for
creating an installation that can be moved to a different directory
without re-compiling. We use this approach for template_dir and the
system wide gitconfig. The Windows installer (msysgit) is an example
for such a setup.
This commit moves common code to a new function system_path(). System
paths that are to be interpreted relative to git_exec_path are passed to
system_path() and the return value is used instead of the original path.
system_path() prefixes a relative path with git_exec_path and leaves
absolute paths unmodified. For example, we now write
Signed-off-by: Steffen Prohaska <prohaska@zib.de> Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at> Signed-off-by: Junio C Hamano <gitster@pobox.com>