doc: improve git diff of texinfo files * .gitattributes: Add rule for *.texi files, with hint on how to use it. Copied from m4, and based on a report by Bruno Haible. Signed-off-by: Eric Blake <ebb9@byu.net>

.deps
Makefile
alloca.h
charset.alias
fnmatch.h
fts.h
getdate.c
getdate.tab.c
getopt.h
lstat.c
poll.h
ref-add.sed
ref-del.sed
search.h
stat.c
stdbool.h
stdint.h
sysexit.h
t-fpending
unlocked-io.h