Only ignore amalgamation files at the root (#641)

Currently, the gitignore tries to ignore src/roaring.c

This actually only works in gitignore because git only considers the gitignore
when it would _add_ a file to be tracked, changes to existing files already in
git are tracked regardless of what's in .gitignore.

Fixes #637

.gitignore

@@ -321,11 +321,12 @@ paket-files/
 __pycache__/
 *.pyc
 
-amalgamation_demo
-amalgamation_demo.c
-amalgamation_demo.cpp
-roaring.c
-roaring.h
-roaring.hh
+# Exclude amalgamation created files
+/amalgamation_demo
+/amalgamation_demo.c
+/amalgamation_demo.cpp
+/roaring.c
+/roaring.h
+/roaring.hh
 
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