Makefile: installing git in cygwin 1.7.0

On platforms with $X, make removes any leftover scripts 'a' from
earlier builds if a new binary 'a.exe' is now built.  However, on
cygwin 1.7.0, 'git' and 'git.exe' now consistently name the same file.
Test for file equality before attempting a remove, in order to avoid
nuking just-built binaries.

This repeats commit 0d768f7 for the installation destdir.

Signed-off-by: Eric Blake <ebb9@byu.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1544,7 +1544,7 @@ ifndef NO_TCLTK
 	$(MAKE) -C git-gui gitexecdir='$(gitexec_instdir_SQ)' install
 endif
 ifneq (,$X)
-	$(foreach p,$(patsubst %$X,%,$(filter %$X,$(ALL_PROGRAMS) $(BUILT_INS) git$X)), $(RM) '$(DESTDIR_SQ)$(gitexec_instdir_SQ)/$p';)
+	$(foreach p,$(patsubst %$X,%,$(filter %$X,$(ALL_PROGRAMS) $(BUILT_INS) git$X)), test '$(DESTDIR_SQ)$(gitexec_instdir_SQ)/$p' -ef '$(DESTDIR_SQ)$(gitexec_instdir_SQ)/$p$X' || $(RM) '$(DESTDIR_SQ)$(gitexec_instdir_SQ)/$p';)
 endif
 	bindir=$$(cd '$(DESTDIR_SQ)$(bindir_SQ)' && pwd) && \
 	execdir=$$(cd '$(DESTDIR_SQ)$(gitexec_instdir_SQ)' && pwd) && \