Added support for file list in single file update

[dhruva]

Hello,
 I wanted to an ability to update existing tags based on incremental
changes. My incremental changes are on a small bunch of files. Here is
the workflow.

1. Create a p4 client (we use perforce with some cool snapshot based
client creation at work)
2. I get a pre-built p4 client with gtags built
3. Edit files and update tags (gtags --single-update file.cpp). Invoke
the command for each file that I have modified
4. Other developers have checked in their code over time
5. I create a new p4 client and import my changes into the new client
6. Again, I update my local tags

When tags are pre-built during build between snapshots, I know the
files that are added/modified/deleted. I wanted to improve the
performance by not calling 'gtags' binary multiple times (once per
file) for an update AND not use '-i' and let 'gtags' stat() all files
to find if deleted.

The following patch implements 2 features:

- "GTAGSLOGICALPATH" for supporting logical path instead of realpath
- treat the file given to --single-update as a single file or a file
containing a list of files
-- It supports optional prefixes in the file list '-' or '+' to avoid
costly stat() over NAS (I work in storage and know the overheads of
stat() to try avoiding it)

Sample file list (4 lines):
-path/to/deleted/file.cpp
+path/to/new/file.cpp
+path/to/modified/file.cpp
path/to/a/file/and/let/stat/determine/if/present/or/missing.cpp


I request feedback and potential acceptance if found good enough.
However, I am going to try getting the binary into our build and
speedup the overall build cycle.

with best regards,
dhruva

update_filelist.patch
Description: Binary data