Makefile: generate "po/git.pot" from stable LOCALIZED_C

Different users may generate a different message template file "po/git.pot". This is because the POT file is generated from "$(LOCALIZED_C)", which is supposed to list all the sources that we extract the strings to be translated from. But "$(LOCALIZED_C)" includes "$(C_OBJ)", which only lists the source files used in the current build for a specific platform and specific compiler conditions. Instead of using "$(C_OBJ)", we use "$(FOUND_C_SOURCES)", which lists all source files we keep track of (or ship in a tarball extract), to form a stable "LOCALIZED_C". We also add "$(SCALAR_SOURCES)", which is part of "$(C_OBJ)" but not included in "$(FOUND_C_SOURCES)". With this update, the newly generated "po/git.pot" will have 30 new entries coming from the following C source files: * compat/fsmonitor/fsm-listen-win32.c * compat/mingw.c * compat/regex/regcomp.c * compat/simple-ipc/ipc-win32.c Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>