Documentation: split "install-man" out from "install"

This makes us more consistent with Git's make targets.  Also add a top
level target "install-man" to forward to Documentation/Makefile.

Signed-off-by: John Keeping <john@keeping.me.uk>
Documentation/Makefile: fix quoting in install-man

Signed-off-by: John Keeping <john@keeping.me.uk>
Documentation/Makefile: allow mandir to be overridden

This matches the behaviour of Git's Documentation/Makefile, which was
changed in commit e14421b9 (Allow INSTALL, bindir, mandir to be set in
main Makefile) so that:

	Makefiles in subdirectories now use existing value of INSTALL,
	bindir, mandir if it is set, allowing those to be set in main
	Makefile or in included config.mak.  Main Makefile exports
	variables which it sets.

Signed-off-by: John Keeping <john@keeping.me.uk>
Documentation/Makefile: add "install-html" target

This uses `git --html-path` to try to find the correct install location
since there is no list of paths for Git to search for HTML
documentation.

Signed-off-by: John Keeping <john@keeping.me.uk>
Documentation: add "info" output

This allows the man page to be viewed using `info` with
`info git-integration`.

Signed-off-by: John Keeping <john@keeping.me.uk>
Merge branch 'install-doc-improvements'

Add HTML and .info documentation install targets and make the existing
man targets more consistent with those in git.git.

* install-doc-improvements:
  Documentation: add "info" output
  Documentation/Makefile: add "install-html" target
  Documentation/Makefile: allow mandir to be overridden
  Documentation/Makefile: fix quoting in install-man
  Documentation: split "install-man" out from "install"