Noiseword "extern" has been removed from function decls in the
header files.
* nd/no-extern:
submodule.h: drop extern from function declaration
revision.h: drop extern from function declaration
repository.h: drop extern from function declaration
rerere.h: drop extern from function declaration
line-range.h: drop extern from function declaration
diff.h: remove extern from function declaration
diffcore.h: drop extern from function declaration
convert.h: drop 'extern' from function declaration
cache-tree.h: drop extern from function declaration
blame.h: drop extern on func declaration
attr.h: drop extern from function declaration
apply.h: drop extern on func declaration