stg branch switch causes unnecessary recompilation

[Andrey Borzenkov <arvidjaar@newmail.ru>]

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{pts/1}% stg version
Stacked GIT 0.8.1
git version 1.1.6
Python version 2.4.2 (#2, Jan 30 2006, 18:33:58)
[GCC 4.0.2 (4.0.2-1mdk for Mandriva Linux release 2006.1)]


Apparently stg branch is implemented as

- - pop all patches
- - do git checkout branch
- - push all patches

unfortunately it results in changing timestamp of all patched files causing 
sometimes long recompilations. What I just had was:

- - I have several local patches for kernel that I keep in separate branches; I 
also have one branch that I pick patches into and that I keep in sync with 
upstream.

- - I pulled 2.6.15.4 stable and recompiled. In the process I noticed a trivial 
error in one file

- - I created new branch, put fix in there, switched to my main branch, picked 
patch and did make. It resulted in recompilation of *everything* that was 
touched by *any* patch.

Sometimes it may be quite time consuming. Notice that git checkout does not 
have this problem - it sets timestamp exactly to the one in index. It would 
be nice if stg did the same.

regards

- -andrey
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