I updated my Chromium source code in my machine and gclient was failing to sync. A similar problem has been mentioned in [1], [2], and [3]. It seems that I messed around with the Chromium project and I ended up detaching its git repositories from their HEADs.

$ gclient sync
Syncing projects: 100% (87/87) src/v8                                      

src/v8 (ERROR)
----------------------------------------
[0:00:04] Started.
----------------------------------------
Error: 78> 
78> ____ src/v8 at c5efc5092fabb0a45351c7b0031b14ed07d3c696
78> 	You have unstaged changes.
78> 	Please commit, stash, or reset.

$ git status
You are in 'detached HEAD' state...

I devised a solution to this problem by dividing into three stages.

1. Remove delete depot_tools and clone new version without changing its location.

   rm -rf depot_tools
   git clone https://chromium.googlesource.com/chromium/tools/depot_tools.git
   

2. Clean all the projects in src directory of the Chromium project by recursively visiting each project. Also checkout the master branch after cleaning:

   export HM=$PWD
   find `pwd` -type d -name ".git" | sed  s/.git$//g |  \
   while read r; do cd $r; git reset --hard HEAD; git checkout master; done
   

3. Update all branches and call gclient sync again

   git rebase-update && gclient sync
   

   If everything goes well, you can rebuild your Chromium code.