contrib/subtree/Makefile: use GIT-VERSION-FILE

GVF is already being used in most/all other makefiles in the project,
and has been for _quite_ a while. Hence, drop file-unique gitver and
replace with GIT_VERSION.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: James Denholm <nod.helm@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
contrib/subtree/Makefile: s/libexecdir/gitexecdir/

$(libexecdir) isn't used anywhere else in the project, while
$(gitexecdir) is the standard in the other appropriate makefiles. Hence,
replace the former with the latter.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: James Denholm <nod.helm@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
contrib/subtree/Makefile: clean up rules to generate documentation

git:Documentation/Makefile establishes asciidoc/xmlto calls as being
handled through their appropriate variables, Hence, change to bring into
congruency with.

Similarly, MANPAGE_XSL exists in git:Documentation/Makefile, while
MANPAGE_NORMAL_XSL does not outside contrib/subtree. Hence, replace
MANPAGE_NORMAL_XSL with MANPAGE_XSL.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: James Denholm <nod.helm@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
contrib/subtree/Makefile: clean up rule for "clean"

git:Documentation/Makefile and others establish "RM ?= rm -f" as a
convention for rm calls in clean rules, hence follow this convention
instead of simply forcing clean to use rm.

subproj and mainline no longer need to be removed in clean, as they are
no longer created in git:contrib/subtree by "make test". Hence, remove
the rm call for those folders.

Other makefiles don't remove "*~" files, remove the rm call to prevent
unexpected behaviour in the future. Similarly, clean doesn't remove the
installable file, so rectify this.

Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: James Denholm <nod.helm@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>