Fix file paths in .repro tar file

Previously, all pathnames were accidentally recorded as the name of
the tar file itself.

Fixes #1227

elf/passes.cc

@@ -1004,7 +1004,7 @@ void write_repro_file(Context<E> &ctx) {
       // We reopen a file because we may have modified the contents of mf
       // in memory, which is mapped with PROT_WRITE and MAP_PRIVATE.
       MappedFile *mf2 = must_open_file(ctx, mf->name);
-      tar->append(path, mf2->get_contents());
+      tar->append(to_abs_path(mf->name), mf2->get_contents());
       mf2->unmap();
     }
   }

test/elf/repro.sh

@@ -18,12 +18,14 @@ $CC -B. -o $t/exe $t/a.o
 $CC -B. -o $t/exe $t/a.o -Wl,-repro
 
 tar -C $t -xf $t/exe.repro.tar
+tar -C $t -tvf $t/exe.repro.tar | grep -q ' exe.repro/.*/a.o'
 grep -q /a.o  $t/exe.repro/response.txt
 grep -q '[ms]old' $t/exe.repro/version.txt
 
 rm -rf $t/exe.repro $t/exe.repro.tar
 
 MOLD_REPRO=1 $CC -B. -o $t/exe $t/a.o
+tar -C $t -tvf $t/exe.repro.tar | grep -q ' exe.repro/.*/a.o'
 tar -C $t -xf $t/exe.repro.tar
 grep -q /a.o  $t/exe.repro/response.txt
 grep -q '[ms]old' $t/exe.repro/version.txt