hashmap: add string interning API

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>

Documentation/technical/api-hashmap.txt

@@ -193,6 +193,21 @@ more entries.
 `hashmap_iter_first` is a combination of both (i.e. initializes the iterator
 and returns the first entry, if any).
 
+`const char *strintern(const char *string)`::
+`const void *memintern(const void *data, size_t len)`::
+
+	Returns the unique, interned version of the specified string or data,
+	similar to the `String.intern` API in Java and .NET, respectively.
+	Interned strings remain valid for the entire lifetime of the process.
++
+Can be used as `[x]strdup()` or `xmemdupz` replacement, except that interned
+strings / data must not be modified or freed.
++
+Interned strings are best used for short strings with high probability of
+duplicates.
++
+Uses a hashmap to store the pool of interned strings.
+
 Usage example
 -------------