Limit some .gitignore patterns

Summary:
These patterns are just meant to ignore stuff in the root directory of
redex, but they weren't properly anchored, so we were ignoring everything under
"tools/redex-all", since "redex-all" was listed as a pattern.  This manifested
hilariously for me as wasting a good 30 minutes wondering why my fancy
.gitignore-aware grep wasn't finding stuff it should have.

Reviewed By: int3

Differential Revision: D3582618

fbshipit-source-id: 534a4b366026d2804342e4658ff06af602dd1d48

.gitignore

@@ -6,25 +6,25 @@
 **/Makefile
 **/Makefile.in
 
-.libs
-aclocal.m4
-autom4te.cache
-cmd.txt
-compile
-config.*
-configure
-depcomp
-install-sh
-libredex.la
-libtool
-ltmain.sh
-m4/libtool.m4
-m4/ltoptions.m4
-m4/ltsugar.m4
-m4/ltversion.m4
-m4/lt~obsolete.m4
-missing
-redex
-redex-all
-stamp-h1
-test-driver
+/.libs
+/aclocal.m4
+/autom4te.cache
+/cmd.txt
+/compile
+/config.*
+/configure
+/depcomp
+/install-sh
+/libredex.la
+/libtool
+/ltmain.sh
+/m4/libtool.m4
+/m4/ltoptions.m4
+/m4/ltsugar.m4
+/m4/ltversion.m4
+/m4/lt~obsolete.m4
+/missing
+/redex
+/redex-all
+/stamp-h1
+/test-driver