Fix potential KeyError in merge_compdb()

[i-ky]

At the point where entry['directory'] is used, 'directory' is definitely not in entry as checked in if two lines above

compiledb/compiledb/__init__.py

Line 88 in b615d52

if 'directory' in entry:

On the other hand, 'file' is guaranteed to be in entry, because gen_key(c) is called only if 'file' in c

compiledb/compiledb/__init__.py

Lines 95 to 96 in b615d52

	orig = {gen_key(c): c for c in compdb if 'file' in c}
	new = {gen_key(c): c for c in new_compdb if 'file' in c}

This looks like a "typo".