gettext: handle GIT_TEXTDOMAINDIR relative to $(prefix)

On Windows, there is no single root directory. And what Git thinks is a
root directory is not a root directory at all: everything is relative to
the location where Git is installed.

To handle this situation better, let's just allow for GIT_TEXTDOMAINDIR
to be a path relative to the (runtime) prefix.

To that end, we have to switch the order in which common-main handles
argv0 and sets up gettext: in order to have access to the runtime
prefix, we need it to be inferred from argv0 already.

This patch also prepares for GIT_LOCALE_PATH to be relative to prefix,
which is the even more important fix.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho