read-cache: fix index corruption with index v4

read-cache: fix index corruption with index v4

ce012deb98 ("read-cache: avoid allocating every ondisk entry when
writing", 2017-08-21) changed the way cache entries are written to the
index file.  While previously it wrote the name to an struct that was
allocated using xcalloc(), it now uses ce_write() directly.  Previously
ce_namelen - common bytes were written to the cache entry, which would
automatically make it nul terminated, as it was allocated using calloc.

Now we are writing ce_namelen - common + 1 bytes directly from the
ce->name to the index.  If CE_STRIP_NAME however gets set in the split
index case ce->ce_namelen is set to 0 without changing the actual
ce->name buffer.  When index-v4, this results in the first character of
ce->name being written out instead of just a terminating nul charcter.

As index-v4 requires the terminating nul character as terminator of
the name when reading it back, this results in a corrupted index.

Fix that by only writing ce_namelen - common bytes directly from
ce->name to the index, and adding the nul terminator in an extra call to
ce_write.

This bug was turned up by setting TEST_GIT_INDEX_VERSION = 4 in
config.mak and running the test suite (t1700 specifically broke).

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>