git-help -i: show info documentation from matching version of git

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -155,6 +155,7 @@ STRIP ?= strip
 prefix = $(HOME)
 bindir = $(prefix)/bin
 mandir = $(prefix)/share/man
+infodir = $(prefix)/share/info
 gitexecdir = $(bindir)
 sharedir = $(prefix)/share
 template_dir = $(sharedir)/git-core/templates
@@ -750,6 +751,7 @@ ETC_GITCONFIG_SQ = $(subst ','\'',$(ETC_GITCONFIG))
 DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
 bindir_SQ = $(subst ','\'',$(bindir))
 mandir_SQ = $(subst ','\'',$(mandir))
+infodir_SQ = $(subst ','\'',$(infodir))
 gitexecdir_SQ = $(subst ','\'',$(gitexecdir))
 template_dir_SQ = $(subst ','\'',$(template_dir))
 htmldir_SQ = $(subst ','\'',$(htmldir))
@@ -798,7 +800,9 @@ git$X: git.o $(BUILTIN_OBJS) $(GITLIBS)
 		$(BUILTIN_OBJS) $(ALL_LDFLAGS) $(LIBS)
 
 help.o: help.c common-cmds.h GIT-CFLAGS
-	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) '-DGIT_MAN_PATH="$(mandir_SQ)"' $<
+	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) \
+		'-DGIT_MAN_PATH="$(mandir_SQ)"' \
+		'-DGIT_INFO_PATH="$(infodir_SQ)"' $<
 
 git-merge-subtree$X: git-merge-recursive$X
 	$(QUIET_BUILT_IN)$(RM) $@ && ln git-merge-recursive$X $@

help.c

@@ -286,6 +286,7 @@ static void show_man_page(const char *git_cmd)
 static void show_info_page(const char *git_cmd)
 {
 	const char *page = cmd_to_page(git_cmd);
+	setenv("INFOPATH", GIT_INFO_PATH, 1);
 	execlp("info", "info", "gitman", page, NULL);
 }