Change default man page path to /usr/share/man

According to FHS,

    http://www.pathname.com/fhs/pub/fhs-2.3.html#USRSHAREMANMANUALPAGES

default man page path is $prefix/share/man.
Signed-off-by: NIsmail Donmez <ismail@pardus.org.tr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -29,7 +29,7 @@ DOC_MAN7=$(patsubst %.txt,%.7,$(MAN7_TXT))
 
 prefix?=$(HOME)
 bindir?=$(prefix)/bin
-mandir?=$(prefix)/man
+mandir?=$(prefix)/share/man
 man1dir=$(mandir)/man1
 man5dir=$(mandir)/man5
 man7dir=$(mandir)/man7