.gitignore: add some rules for tagging programs

Often, we miss committing new files, and 'arc diff' is supposed to warn
us about this. Unfortunately, because of the spurious output of the
command (due to unignored untracked files), we tend to ignore it and
lose information.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225588 91177308-0d34-0410-b5e6-96231b3b80d8

.gitignore

@@ -24,6 +24,13 @@
 #==============================================================================#
 # Explicit files to ignore (only matches one).
 #==============================================================================#
+# Various tag programs
+/tags
+/TAGS
+/GPATH
+/GRTAGS
+/GSYMS
+/GTAGS
 .gitusers
 autom4te.cache
 cscope.files