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>