Bug: rebase -i creates committer time inversions on 'reword'

[Johannes Sixt]

I just noticed that all commits in a 70-commit branch have the same
committer timestamp. This is very unusual on Windows, where rebase -i of
such a long branch takes more than one second (but not more than 3 or
so thanks to the builtin nature of the command!).

And, in fact, if you mark some commits with 'reword' to delay the quick
processing of the patches, then the reworded commits have later time
stamps, but subsequent not reworded commits receive the earlier time
stamp. This is clearly not intended.

Perhaps something like this below is needed.

diff --git a/ident.c b/ident.c
index 327abe557f..2c6bff7b9d 100644
--- a/ident.c
+++ b/ident.c
@@ -178,8 +178,8 @@ const char *ident_default_email(void)
 
 static const char *ident_default_date(void)
 {
-       if (!git_default_date.len)
-               datestamp(&git_default_date);
+       strbuf_reset(&git_default_date);
+       datestamp(&git_default_date);
        return git_default_date.buf;
 }