[MERGE] Accelerate merge by skipping file existence check when merging execute bit

[John Arbash Meinel]

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Aaron Bentley wrote:
| Hi all,
|
| This patch speeds up merge by avoiding unnecessary file-existence tests.
|
| During merge, we may wish to set or unset the execute bit, but if the
| file does not exist, we must not.  Currently, we check whether the file
| exists whether or not we will set it.  This can happen for many files in
| the tree.  This patch avoids unnecessary existence checks.
|
| This accelerates execute bit merging, and also accelerates conflict
| resolution.  Conflict resolution scales with the number of files that
| have been referenced.  The fewer the number of referenced files, the
| faster the resolution.  The existence check was causing files to be
| referenced that otherwise would not have been, so skipping it speeds up
| conflict resolution as well.
|
| Aaron

BB:approve

Do you have any numbers to back this up?

Is it only for files involved in the merge? (So if there were 10 changed files
in a tree of 10,000 this would only skip the existence check on 9 files if the
10th had an exec bit changed.)

John
=:->
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