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

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>