[Svnmerge] Slow generation of commit file

[Giovanni Bajo]

Hello,

Daniel Berlin complained that generating the commit file might take even longer
than doing the merge. He is actually right: construct_merged_log_message
invokes a distinct "svn log" for each and every revision! That's something I
had totally missed. When I said "you can disable commit file generation by
using /dev/null" I was wrong: disabling commit file also saves (possible
multiple) svn invokations, so there ought to be a way to do it.

I have committed (in my repository) a version which adds "-f-" as a way to shut
off commit file generation. I'll be porting it to the SVN repository soon (and
I'll be dropping my repository soon as well, when everything is merged over).
BTW, in my repository I also implemented a real test framework for svnmerge,
for which I have constructed real testcases for basic commands. That's great
for development!

I believe we should tweak construct_merged_log_message so to invoke "svn log"
only once, including the whole range of revisions we need a log for. Some past
experiments have shown me that "svn log" has slow setup and fast execution, so
it's much better to ask more data, rather than calling it with more resolution.
Especially for normal cases, I don't expect wild ranges in revisions being
merged, so it looks like the best solution and the easiest to implement.
Comments?

Giovanni Bajo