Fix in-place editing functions in convert.c

* crlf_to_git and ident_to_git:

Don't grow the buffer if there is enough space in the first place.
As a side effect, when the editing is done "in place", we don't grow, so
the buffer pointer doesn't changes, and `src' isn't invalidated anymore.

Thanks to Bernt Hansen for the bug report.

* apply_filter:

Fix memory leak due to fake in-place editing that didn't collected the
old buffer when the filter succeeds. Also a cosmetic fix.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>