Interning short strings with high probability of duplicates can reduce the
memory footprint and speed up comparisons.
Add strintern() and memintern() APIs that use a hashmap to manage the pool
of unique, interned strings.
Note: strintern(getenv()) could be used to sanitize git's use of getenv(),
in case we ever encounter a platform where a call to getenv() invalidates
previous getenv() results (which is allowed by POSIX).
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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