Makefile: generate Git(3pm) as dependency of the 'doc' and 'man' targets

Makefile: generate Git(3pm) as dependency of the 'doc' and 'man' targets

Since commit 20d2a30f8f (Makefile: replace perl/Makefile.PL with
simple make rules, 2017-12-10), the Git(3pm) man page is only
generated as an indirect dependency of the 'install-doc' and
'install-man' Makefile targets.  Consequently, if someone runs 'make
man && sudo make install-man' (or their 'doc' counterparts), then
Git(3pm) will be generated as root, and the resulting root-owned files
and directories will in turn cause the next user-run 'make clean' to
fail.  This was not an issue in the past, because Git(3pm) was
generated when 'make all' descended into 'perl/', which is usually not
run as root.

List Git(3pm) as a dependency of the 'doc' and 'man' Makefile targets,
too, so it gets generated by targets that are usually built as
ordinary users.

While at it, add 'install-man-perl' to the list of .PHONY targets.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Acked-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>