stop installing git-tar-tree link

When the built-in "git tar-tree" command (a thin wrapper around "git
archive") was removed in 925ceccf (tar-tree: remove deprecated
command, 2013-11-10), the build continued to install a non-functioning
git-tar-tree command in gitexecdir by mistake:

	$ PATH=$(git --exec-path):$PATH
	$ git-tar-tree -h
	fatal: cannot handle tar-tree internally

The list of links in gitexecdir is populated from BUILTIN_OBJS, which
includes builtin/tar-tree.o to implement "git get-tar-commit-id".
Rename the get-tar-commit-id source file to builtin/get-tar-commit-id.c
to reflect its purpose and fix 'make install'.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -586,7 +586,6 @@ BUILT_INS += git-cherry$X
 BUILT_INS += git-cherry-pick$X
 BUILT_INS += git-format-patch$X
 BUILT_INS += git-fsck-objects$X
-BUILT_INS += git-get-tar-commit-id$X
 BUILT_INS += git-init$X
 BUILT_INS += git-merge-subtree$X
 BUILT_INS += git-show$X
@@ -929,6 +928,7 @@ BUILTIN_OBJS += builtin/fmt-merge-msg.o
 BUILTIN_OBJS += builtin/for-each-ref.o
 BUILTIN_OBJS += builtin/fsck.o
 BUILTIN_OBJS += builtin/gc.o
+BUILTIN_OBJS += builtin/get-tar-commit-id.o
 BUILTIN_OBJS += builtin/grep.o
 BUILTIN_OBJS += builtin/hash-object.o
 BUILTIN_OBJS += builtin/help.o
@@ -980,7 +980,6 @@ BUILTIN_OBJS += builtin/show-ref.o
 BUILTIN_OBJS += builtin/stripspace.o
 BUILTIN_OBJS += builtin/symbolic-ref.o
 BUILTIN_OBJS += builtin/tag.o
-BUILTIN_OBJS += builtin/tar-tree.o
 BUILTIN_OBJS += builtin/unpack-file.o
 BUILTIN_OBJS += builtin/unpack-objects.o
 BUILTIN_OBJS += builtin/update-index.o