branch: reset instead of release a strbuf

branch: reset instead of release a strbuf

Our documentation advises to not re-use a strbuf, after strbuf_release
has been called on it. Use the proper reset instead.

Currently 'strbuf_release' releases and re-initializes the strbuf, so it
is safe, but slow. 'strbuf_reset' only resets the internal length variable,
such that this could also be accounted for as a micro-optimization.

Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>