Let git-help prefer man-pages installed with this version of git

Prepend $(prefix)/share/man to the MANPATH environment variable before
invoking 'man' from help.c:show_man_page().  There may be other git
documentation in the user's MANPATH but the user is asking a specific
instance of git about its own documentation, so we'd better show the
documentation for _that_ instance of git.

Signed-off-by: Sergei Organov <osv@javad.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -154,6 +154,7 @@ STRIP ?= strip
 
 prefix = $(HOME)
 bindir = $(prefix)/bin
+mandir = $(prefix)/share/man
 gitexecdir = $(bindir)
 sharedir = $(prefix)/share
 template_dir = $(sharedir)/git-core/templates
@@ -746,6 +747,7 @@ ETC_GITCONFIG_SQ = $(subst ','\'',$(ETC_GITCONFIG))
 
 DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
 bindir_SQ = $(subst ','\'',$(bindir))
+mandir_SQ = $(subst ','\'',$(mandir))
 gitexecdir_SQ = $(subst ','\'',$(gitexecdir))
 template_dir_SQ = $(subst ','\'',$(template_dir))
 prefix_SQ = $(subst ','\'',$(prefix))
@@ -792,7 +794,8 @@ git$X: git.o $(BUILTIN_OBJS) $(GITLIBS)
 	$(QUIET_LINK)$(CC) $(ALL_CFLAGS) -o $@ git.o \
 		$(BUILTIN_OBJS) $(ALL_LDFLAGS) $(LIBS)
 
-help.o: common-cmds.h
+help.o: help.c common-cmds.h GIT-CFLAGS
+	$(QUIET_CC)$(CC) -o $*.o -c $(ALL_CFLAGS) '-DGIT_MAN_PATH="$(mandir_SQ)"' $<
 
 git-merge-subtree$X: git-merge-recursive$X
 	$(QUIET_BUILT_IN)$(RM) $@ && ln git-merge-recursive$X $@

help.c

@@ -239,6 +239,27 @@ void list_common_cmds_help(void)
 	}
 }
 
+static void setup_man_path(void)
+{
+	struct strbuf new_path;
+	const char *old_path = getenv("MANPATH");
+
+	strbuf_init(&new_path, 0);
+
+	/* We should always put ':' after our path. If there is no
+	 * old_path, the ':' at the end will let 'man' to try
+	 * system-wide paths after ours to find the manual page. If
+	 * there is old_path, we need ':' as delimiter. */
+	strbuf_addstr(&new_path, GIT_MAN_PATH);
+	strbuf_addch(&new_path, ':');
+	if (old_path)
+		strbuf_addstr(&new_path, old_path);
+
+	setenv("MANPATH", new_path.buf, 1);
+
+	strbuf_release(&new_path);
+}
+
 static void show_man_page(const char *git_cmd)
 {
 	const char *page;
@@ -254,6 +275,7 @@ static void show_man_page(const char *git_cmd)
 		page = p;
 	}
 
+	setup_man_path();
 	execlp("man", "man", page, NULL);
 }